This month, I’ve decided to do a deep-dive into wearable skin temperature trackers. It’s my hope that this Info Hub will be a helpful, comprehensive resource for you to browse through should you be interested in purchasing a wearable skin temperature tracker for Fertility Awareness Based Method (FABM) charting.

Without fail, one of the most common questions I get in my direct messages on social media is “what do you think about so-and-so wearable tracking/fertility device or app?”.

This is no surprise given the growing popularity of fertility tracking devices, and the saturation of the femtech* market, with more new products and innovations being released each year.

Disclaimers:

• Details are correct to the best of my knowledge as at the time of writing this article in October 2022; however, the femtech industry moves quickly with constant innovation and device/app upgrades. Keep this in mind and always do your own due diligence if you’re interested in purchasing any sort of femtech device.
• I am not a “tech writer”. I use very minimal technology in my own FABM charting practise. Not only that but I also prefer to paper chart, and I manufacture and sell beautiful charting journals which you can browse here. I’ve aggregated and summarised as much relevant info as I could find in this information hub, but I am not an authority or expert on emerging FABM charting technologies.
• I am not trained in statistical analysis or study analysis, so where I link to/discuss studies you are reading my personal opinion. If you’re a FABM enthusiast statistician I’m sure you can glean some interesting insights from some of the studies mentioned!

In addition, this information hub barely scratches the surface when it comes to wearable temperature trackers and is simply designed as a springboard for you to do your own further investigation on any topics that interest you. If you see something that is incorrect or should be changed, please let me know in the comments below.

Alright, let’s dive in!

*The term “femtech” is short for “female technology”. It refers to tools, technologies, services and software that aim to address women’s/menstruators’ health issues.

Click on each of the headings below to learn more. Start at the top and work your way down.

Basal body temperature (BBT) thermometers

Not all thermometers are created equally.

Basal body temperature (BBT) thermometers are designed to pick up very small fluctuations in body temperatures. BBT thermometers are more accurate than standard fever thermometers.

Most Symptothermal Methods require that you use a basal body temperature thermometer that displays two decimal places.

Here are some important parameters to keep in mind when you are searching for a thermometer to use with Symptothermal Methods:

Measuring accuracy and tolerance:
Measuring accuracy refers to how close your thermometer will measure to a known temperature. Measuring accuracy is usually stated as a tolerance which explains how far either side of a known temperature your thermometer might measure. BBT thermometers usually have an accuracy of +/-0.05°C / 0.1°F.

This means that if the known temperature is 36.53°C, your BBT thermometer may give you a reading anywhere between 36.48°C and 36.58°C.

Display resolution:
Display resolution refers to how many decimal places your thermometer/device displays. Basal body temperature thermometers should have a display resolution of one hundredth of a degree (0.01°C / 0.01°F).

Precision:
Precision refers to how consistent the results are when the measurement is repeated. When it comes to charting with the Symptothermal Method, precision is more important than accuracy. A highly precise thermometer might be slightly inaccurate, but the error in accuracy will be repeated every day of the menstrual cycle, meaning that we can confidently identify our temperature shift (knowing that a shift is not caused by an imprecise thermometer reading, but by our true rise in temperature after ovulation).

In other words, even if your BBT thermometer is slightly inaccurate this will not affect your charts so long as your thermometer is precise, and the error is consistent across the entire chart.

Different types of temperatures

To begin with, it’s helpful to know a few terms.

Core body temperatures refer to internal temperatures from our core (usually obtained via oral, vaginal or rectal sites for FABM charters).

Distal skin temperatures usually refer to skin temperatures taken from the hands and feet (wrists are included).

Proximal skin temperatures usually refer to skin temperatures taken from closer proximity to our core (such as our upper arms/thighs and stomach).

Basal body temperature (BBT) is our core body temperature at its lowest state while we are in a state of rest. This usually occurs at around 4am each morning. Basal body temperatures are important for Symptothermal Method charters because they usually rise for two weeks after ovulation has occurred, until our next period begins. This rise in temperature is due to a hormone called progesterone, which we release in large quantities in the two weeks after ovulation. Charting our temperatures (in conjunction with our cervical mucus) helps us to confirm that ovulation has passed. To learn how to do this, visit the Instructor Directory or visit the Self-Teaching page on this website.

Waking temperatures (either oral, vaginal or rectal) are used as a close approximation of true basal body temperature for Symptothermal Method charters. Taking our temperature as soon as we wake up saves us having to wake up at 4am each day.

What happens to our skin temperatures when we fall asleep?

Melatonin is a hormone that helps us to fall asleep, and our bodies naturally produce more of it in the evening as darkness approaches.

The melatonin we produce sends signals to our distal skin regions (hands and feet) that create a rise in blood flow. This rise in blood flow is due to vasodilation (expansion of the veins that hold our blood).

With more blood flowing through these veins, we lose heat through the skin of our extremities.

The heat loss in our extremities gradually lowers our core body temperature during the night. Our core body temperature is usually lowest at around 4am in the morning.

What is the difference between hand and upper-arm skin temperatures?

During waking hours, our distal temperatures (hands and feet) are lower than our proximal temperatures (upper arms/thighs and stomach etc).

After we fall asleep, our distal temperatures rise significantly (via the process of vasodilation mentioned above). This rise causes distal temperatures to then match proximal temperatures closely throughout the night. After waking, distal temperatures then drop lower once again.

What does this mean for FABM charting? During the night while asleep, distal temperatures closely match proximal temperatures. Many FABM charters already successfully use proximal upper-arm temperatures via the Tempdrop device to accurately visualise and assess their temperature shifts. With the right technology, FABM charters will likely soon be able to successfully utilise distal skin temperatures for FABM charting, too.

What is the difference between core body temperature and skin temperatures?

True core body temperature tends to follow a general circadian rhythm which results in a smooth rise and fall of temperatures every 24 hours. Core body temperature is generally lowest at around 4am in the morning when humans are naturally in a deep sleep. Following this, it gradually rises throughout the day to peak at around 6-8pm before falling again.

Distal and proximal skin temperatures do not follow this same circadian rhythm. Instead, they share an inverse relationship during wake times, before slowly converging to rise rapidly as sleep commences, before dipping around 2am and slowly rising back up again by around 5-6am.

If core body temperature and skin temperatures don't follow the same pattern throughout the day, how can they be used for charting?

When we are charting our temperatures with a Fertility Awareness-Based Method, we are interested in identifying infradian rhythms, as opposed to circadian rhythms. Circadian rhythms refer to 24-hour cycles. The roughly 28-day process of the menstrual cycle is an infradian rhythm.

Skin temperatures do not mirror core body temperatures during a single 24 hour circadian rhythm cycle. Despite this, they are still influenced by our changing hormones across our infradian menstrual cycle. They show a shift from lower temperatures prior to ovulation, to higher temperatures after ovulation. This means they can still be used to identify temperature shifts with FABMs (although to what level of contraceptive effectiveness, we don’t yet have a lot of evidence).

Some studies that have explored the biphasic properties of skin temperature across the menstrual cycle include:

• Chen W, Kitazawa M, Togawa T. Estimation of the biphasic property in a female's menstrual cycle from cutaneous temperature measured during sleep. Ann Biomed Eng. 2009;37(9):1827.


• Kräuchi K, Konieczka K, Roescheisen-Weich C, Gompper B, Hauenstein D, Schoetzau A, et al. Diurnal and menstrual cycles in body temperature are regulated differently: a 28-day ambulatory study in healthy women with thermal discomfort of cold extremities and controls. Chronobiol Int. 2014;31(1):102–13.


• Maijala, A., Kinnunen, H., Koskimäki, H. et al. Nocturnal finger skin temperature in menstrual cycle tracking: ambulatory pilot study using a wearable Oura ring. BMC Women's Health 19, 150 (2019).


• Alzueta E, de Zambotti M, Javitz H, Dulai T, Albinni B, Simon KC, Sattari N, Zhang J, Shuster A, Mednick SC, Baker FC. Tracking Sleep, Temperature, Heart Rate, and Daily Symptoms Across the Menstrual Cycle with the Oura Ring in Healthy Women. Int J Womens Health. 2022 Apr 8;14:491-503.

What does this mean for FABM charting? Considering the Tempdrop device (measuring proximal temperatures), has been shown* to mirror core body temperature changes across the menstrual cycle, it's likely that other wearable skin temperature trackers (both distal and proximal), will be able to do the same. For instance, the Oura Gen3 Ring has been shown to produce comparable results to oral temping for Natural Cycles app users (read more about this in the Oura section below). It’s important to note that most wearable skin temperature thermometers will need to utilise multiple sensors and an algorithm to filter out environmental and movement disturbances to provide an accurate temperature each morning.

*Unfortunately, we do not have high quality clinical trial evidence of the accuracy of Tempdrop temperatures in the context of Symptothermal Method charting effectiveness. At present, we are relying more on the “social proof” of the many thousands of people currently using the Tempdrop to successfully identify biphasic temperature shifts during the menstrual cycle. The biphasic shift to a higher temperature is caused by the event of ovulation that occurs roughly 12-14 days prior to the onset of menstruation. Hopefully in future we will have more evidence not just on the Tempdrop but on all other skin-sensor wearables.

Why do we need multiple sensors and algorithms for accurate skin temperatures?

Skin temperatures are subject to higher levels of variability than our core body temperature. This is because our skin is more exposed to changes in environmental temperatures. Plus, the blood vessels in our skin are designed to rapidly contract or expand to conserve or release heat, thereby maintaining our relatively stable core body temperature.

What does this mean? Most skin temperature-collection devices will need to filter temperatures through an algorithm that also tracks data collected about:

• Environmental temperatures
• Your body position
• Your activity level
• Your sleep status

As an example, the Tempdrop thermometer provides data based on proximal skin temperatures and is worn under the armpit close to the axillary artery. It measures skin temperatures with one sensor, ambient environmental temperatures with another sensor, and utilises a 3-axis accelerometer to track your body position, activity and sleep status. Data is then filtered through an algorithm to provide the most accurate skin temperature reading from the night. Most other skin-wearable thermometers have a similar approach.

Now that you’ve read through all of the above topics, you will have a better understanding of the relationship between our core body temperature, skin temperatures (both proximal and distal), and how these can be used for FABM charting.

If you’re curious to know more about some of the available products when it comes to wearable skin-temperature tracking devices, read on! For each device mentioned below, I have outlined key features plus my own personal opinion on its suitability for use with Symptothermal Method charting.

Tempdrop

Device: Tempdrop
Price: From USD $200
Type of skin temperatures (proximal or distal): Proximal
Skin site: Worn in an armband to collect underarm temperatures from near to the axillary artery.
Time worn: Worn only while sleeping
EMF exposure: The Tempdrop only emits EMFs while syncing with the Tempdrop app in the morning. EMFs are not released while being worn.
Associated app: Tempdrop syncs directly with the Tempdrop app. To view your temperatures, you must sync your Tempdrop with the Tempdrop app.
3rd Party integrations: Once you sync your Tempdrop with the Tempdrop app to retrieve your temperature, you can also auto-retrieve it into these additional apps on your phone: Chart Neo app, Read Your Body app, and OvuView app
Subscriptions required: None. You can choose to pay an annual subscription to the premium version of the Tempdrop app to access sleep data and automatic chart markings*.
Type of temperature data: Absolute values
Number of decimals: Two decimal places (.00)
Data points provided:

• Temperature
• Sleep data

My take:
The Tempdrop has the most “social proof” of any wearable skin temperature tracking device. They have over 10,000 users in their Facebook group and have been in use by FABM charters since 2017 now. I personally have successfully used the Tempdrop throughout my postpartum charting journey and am very happy with it.

I like the fact that the Tempdrop is only worn at night as I don’t like wearing things during the daytime (the only jewellery that I can wear 24-7 is a necklace, and even then I prefer taking it off while I shower). I like the fact that the Tempdrop is worn on the upper arm, as I don’t like wearing watches or rings while I sleep, because I sleep with my hands up near my face. This is just a personal idiosyncrasy though, and I’m sure most other people have no problem with watches and rings while sleeping!

I do sometimes wonder if the Tempdrop may be slightly *too* sensitive. In my personal experience, I believe it sometimes picks up the miniscule temperature rise that occurs during ovulation (there is a small amount of progesterone that is released during ovulation). With Sensiplan/NFPTA temperature rules (which make it relatively easy and straightforward to mark temperature shifts), I think this can sometimes result in a temperature shift being marked a day or two earlier than it would with oral temping. It’s possible that TCOYF charters (with their raised coverline), might not share this issue. Again, this is just my hypothesis based on a couple of my own personal charts. For Symptothermal Method users, cervical mucus observations should always be acting as a safety double-check regardless.

The Tempdrop device itself could probably benefit from some additional research and development to make it more user-friendly (especially when it comes to battery changes). I am glad that the algorithm no longer updates the most recent three temperatures, as this previously used to extend the time it takes to confirm a temperature shift in the luteal phase.You can read more of my thoughts about the Tempdrop here. I’m also an affiliate for the Tempdrop device and you can use my code TDLOVE29 to save 12% if you would like to purchase one.

At this point, I believe the Tempdrop remains the best choice for Symptothermal Method charters who cannot obtain oral/vaginal/rectal temperatures for some reason. That said, technologies are changing quickly - all it would take is for one of the other wearables listed here to begin showing accurate, absolute temperature values each morning and Tempdrop would have some serious competition (and I think that once BBRing is released, that will be the case!).

*I do not recommend anyone rely on automatic chart markings to evaluate their charts, especially if you are genuinely wanting to prevent unintended pregnancy. It's generally preferable to learn a method and understand how to mark your own charts.

Oura Gen3 Ring

Device: Oura Gen3
Price: From USD $350
Type of skin temperatures (proximal or distal): Distal
Skin site: Worn in a ring to collect temperatures from the index, middle or ring finger.
Time worn: Recommended to be worn continuously. The Oura Gen3 ring is only taken off to charge, and battery can last up to 7 days. Despite this, Oura states that you can choose to wear it only at night if you prefer.
EMF exposure: The Oura Gen3 ring continuously emits low levels of electric and magnetic fields. The Oura Gen3 ring’s SAR level is 0.0003 W/kg. You can enable airplane mode inside the Oura app whenever you don’t need to look at your data (i.e. while you are sleeping).
Associated app: Oura app
3rd Party integrations: Apple Health, Google Fit, Strava, Natural Cycles (keep reading for my thoughts on Natural Cycles).
Subscriptions required: If you purchase a Gen3 Oura Ring, Oura Membership costs between 5.99 and 6.99 USD per month, depending on where you live in the world. Without a membership, you can still export your Oura data into CSV or JSON format from their web portal.
Type of temperature data: Temperatures are shown as fluctuations (+/-) from your personal “baseline temperature”. This is less useful for FABM charting, as we need an actual temperature value to plot on the graph section of our chart each morning.
Number of decimals: One decimal place (.0)
Data points provided:

• Temperature
• Sleep data
• Respiration/blood oxygen levels
• Resting heart rate
• Heart rate variability

My take:
I don’t really enjoy wearing jewellery on my hands (I almost never wear rings), so my immediate take is that it’s unlikely I would want to be wearing a ring 24-7. I also prefer products that I can buy outright, as opposed to subscription-based (especially with the cost of living rising a lot lately).

The only way currently to see an absolute temperature value each day is by using the Natural Cycles’ app. Read more here on why I don’t recommend Natural Cycles app for those who have a serious need to prevent pregnancy.

Interestingly, in Natural Cycles’ 510(k) premarket submission to the FDA to demonstrate that Oura temperatures are just as safe and effective as oral temperatures with the Natural Cycles app, they state: “Compared to the two-decimal place oral thermometer, the Natural Cycles algorithm provides additional 1.6 green days (not fertile) in the luteal phase of the menstrual cycle when the input temperature was from the Oura ring, without increasing the risk of unintended pregnancy”. I am interested to read more about this, but I cannot find the clinical study of the Oura Ring + Natural Cycles app. My initial reaction is that I am sceptical about this statement. Natural Cycles is already known for “confirming” temperature shifts a day or so earlier than Symptothermal Methods typically do (on the second day of a raised temperature for instance). For Oura Ring temperatures to then subtract an additional 1.6 days off the time it is taking Natural Cycles to confirm a temperature shift, gives me some pause for thought. Again, if you find this study please let me know – I searched high and low on the internet but could find no trace of it apart from its mention in Natural Cycles’ 510(k) FDA submission.

One thing that I do consider a drawcard is the absolute wealth of additional biometric information that Oura ring tracks and provides. If you’re going to spend a lot of money on a wearable device, this is definitely a beautiful premium option.

Overall, I think the Oura ring may have fantastic potential for future charters, provided they are able to begin delivering absolute temperature values (as opposed to fluctuation trends). This would mean FABM charters could use Oura temperatures in their chosen charting app or on their paper charts.

Studies (not an exhaustive list):
It is great to see a company such as Oura dedicating resources toward studying the accuracy of their device for menstrual cycle tracking. Despite this, I have found it difficult to access studies on exactly how accurate Oura temperatures are in comparison to oral waking temperatures for FABM charting. Data/studies that would be most relevant seem to be behind paywalls (Maijala et al., 2018) or I simply could not find them on the internet (Natural Cycles’ clinical study referenced in their 510(k) submission for FDA approval).

In saying this, Oura is the only company that is forthcoming with accuracy/precision figures on their website (this data is not available publicly for any other wearable skin temperature tracker that I researched). According to the Oura website, Oura is proven under laboratory conditions to be accurate to within 0.36°C (0.648°F). Remember that precision is arguably more important for FABM charting. Oura is proven under laboratory conditions to be precise to 0.13°C (0.234°F). It’s important to note that these are laboratory results and may not extend to real-world conditions.

It seems that if we really want reassurance of the comparability between Oura versus oral temperatures for FABM charting, we will need to engage in citizen science and make these comparison charts ourselves. This has definitely already begun in the FABM community, and as an example you can check out a comparison chart by Sensiplan teacher trainee Emma Lukkarila on her Instagram here.

You can also check out a great comparison between oral and Oura temperatures by fellow FABM Instructor Leslie Rewis in the FAM Support Facebook group here. Leslie's comparison is a great example of how distal temperature shifts seem to be more pronounced than oral/vaginal/rectal temperature shifts.

Maijala A, Kinnunen H, Koskimäki H, Jämsä T and Kangas M. Comparison of menstrual phases with nocturnal temperature of the Oura ring and oral body temperature at wake-up. World Congress on Medical Physics and Biomedical Engineering; 2018; Prague, Czech Republic: IUPESM; 2018:312–313.

• As far as I can see, this data actually compares Oura temperatures against oral temperatures, but is only available if you purchase the linked book for between AU $280 - $500. If anyone else has access to these findings I would be curious to read them: https://www.amazon.com/Congress-Medical-Physics-Biomedical-Engineering/dp/9811090378

Maijala, A., Kinnunen, H., Koskimäki, H. et al. Nocturnal finger skin temperature in menstrual cycle tracking: ambulatory pilot study using a wearable Oura ring. BMC Women's Health 19, 150 (2019). This study is full of interesting info, but three things that stood out to me were:

1. This study found that the accuracy of the Oura algorithm increased when the two study subjects with a BMI of over 30 were excluded from the calculations. Study authors state that a BMI of over 30 “is a potential confounder affecting distal skin temperature.” Other studies have shown that obesity causes distal temperatures to rise, to account for the insulating effect of a greater layer of subcutaneous abdominal fat. These higher distal temperatures allowed study participants to release heat and therefore regulate their core body temperature. Is it possible that a higher baseline distal temperature might mute the post-ovulatory rise in distal temperatures? I would love to see more studies on this in future (for both distal and proximal temperatures), as I have also heard some Tempdrop users speculate that their higher BMI may be impacting the accuracy of their temperature readings.
2. Interestingly, the study authors also mention that smoking can impact skin temperature values. As the study was rather small (only 22 participants total) and none of them were smokers, it’s unclear whether this would be relevant for FABM charters.
3. A third point which I found interesting is that distal skin temperatures showed a greater rise than oral temperatures in the luteal phase. Study authors state that the “Temperature difference [between follicular and luteal phases] was higher with skin temperatures than with oral temperatures.” On average, skin temperatures were 0.07 °C higher in the luteal phase. This gives me pause for thought as all Symptothermal temperature shift evaluation rules are based on oral/vaginal/rectal temperatures. It’s possible that the slightly higher rise in finger skin temperatures in the luteal phase might result in some temperature shifts being marked as “confirmed” earlier than they would with an actual FABM method. This is purely my own conjecture, but I think it would be an interesting topic for further study.

Grant, A.D., Newman, M. & Kriegsfeld, L.J. Ultradian rhythms in heart rate variability and distal body temperature anticipate onset of the luteinizing hormone surge. Sci Rep 10, 20378 (2020).

• This is probably one of the most interesting studies based on Oura temperatures that I have seen so far. Researchers found that changes over 2-5 hours in distal temperatures and heart rate variability predicted the onset of the luteinising hormone surge two days in advance for 100% of study participants. Fascinating! I imagine this could have notable benefit for real-time fertile window information for people who are trying to conceive, but would not provide enough advance warning of approaching ovulation for those trying to prevent pregnancy.

Alzueta E, de Zambotti M, Javitz H, Dulai T, Albinni B, Simon KC, Sattari N, Zhang J, Shuster A, Mednick SC, Baker FC. Tracking Sleep, Temperature, Heart Rate, and Daily Symptoms Across the Menstrual Cycle with the Oura Ring in Healthy Women. Int J Womens Health. 2022 Apr 8;14:491-503.

• This study did not deliver any overly interesting data on Oura temperatures for menstrual cycle tracking, apart from showing that temperatures tended to be lower around ovulation, and then higher during the luteal phase (as expected). The study was not designed to test accuracy in comparison to oral temperatures and seemed to be more focused on sleep data.

Natural Cycles has studied Oura temps versus their oral basal temperatures and this is mentioned in their 510(k) FDA submission; however, I cannot find the actual clinical study. You can read their 510(k) submission here.

BBRing by Femtek

Device: BBRing by Femtek
Price: First round on Kickstarter sold out, currently open to waitlist for next round of shipping. Price likely approx. USD $185
Type of skin temperatures (proximal or distal): Distal
Skin site: Worn in a ring to collect temperatures from the index finger.
Time worn: Only while sleeping
EMF exposure: The BBRing can be worn in “offline mode” overnight, reducing exposure to EMFs. Specific SAR levels are not provided, but the Femtek website states that "Our device has passed all EMC testing for the markets in which we are able to ship to, and due the ability to use the device in offline mode overnight, our EMF testers weren’t able to pick anything up on our readers."
Associated app: Basal Body Ring app
3rd Party integrations: Apple/Garmin health kit
Subscriptions required: No subscriptions required
Type of temperature data: Absolute values
Number of decimal places: Two decimal places (.00)
Data points provided:

• Temperature data
• Heart rate
• Heart rate variability

My take:
The BBRing has the potential to be a great addition to the FABM charting toolbox, because it will provide absolute temperature values to two decimal places. This means that the data provided will hopefully be comparable with Tempdrop, and you won’t need to do any calculations/conversions to get an absolute value for your chart (like you would if you were relying on the Oura Gen3 Ring or the Apple Watch).

Again, I don’t personally enjoy wearing jewellery so I would probably prefer the placement of the Tempdrop on the upper arm as opposed to a ring on my finger; however, I appreciate that this device only needs to be worn at night time.

I also like the fact that this device is made by a small Australian company who is prioritising menstrual cycle tracking – this is not simply another large multinational corporation adding on temperatures as an afterthought to try and capitalise on the femtech market. Olivia Orchowski (the founder), reached out to me in mid-2020 with questions on BBT for FABM charting. She was very responsive and open to feedback that would benefit the FABM charting community.

I’m excited to test out the BBRing myself. I am hopeful that it could hold the perfect mix of affordability, accuracy, and user-friendliness for FABM charting.

PS: Be sure to expand the Oura Gen3 Ring info section to read about some interesting study findings that may have applicability to the accuracy of the BBRing, too.

Apple Watch Series 8 / Apple Watch Ultra

Device: Apple Watch Series 8 / Apple Watch Ultra
Price: From USD $390 / From USD $805
Type of skin temperatures (proximal or distal): Distal
Skin site: Worn on the left or right wrist (dorsal side) to collect wrist temperatures
Time worn: Can be worn continuously, or only overnight while sleeping if you are only wanting to use the Apple Watch for temperature tracking for charting.
EMF exposure: EMF levels for the Apple Watch Series 8 are not yet available on the Apple website; however, I can see that the Apple Watch Series 7 has a SAR level of 0.40 W/kg. This is significantly higher than the Oura Gen3 ring’s SAR level of 0.0003 W/kg.
Associated app: The iPhone Health App is used to access the nightly wrist temperatures. The Apple Cycle Tracking app is also available if you would like to make use of Apple’s period predictions. Unfortunately, Apple’s Cycle Tracking app doesn’t show a chart with enough detail to be used for FABM charting.
3rd Party integrations: There are a wide range of third-party apps that can be downloaded onto the Apple Watch. These include internet browsers, to sleep trackers, to heart activity trackers, to weather trackers, podcast players and so much more. Despite this, I am unaware of any 3rd party integrations that work with the Apple Watch temperatures for FABM charting.
Subscriptions required: No subscriptions required
Type of temperature data: Absolute temperature
Number of decimals: Two decimal places (.00)
Data points provided:

• Sleep data
• Temperature data
• Blood oxygen data
• Heart rate data

My take:
I like the fact that the Apple Watch does not need to be worn continuously, and it also does not require a subscription which is helpful given the rising costs of living. In addition, Apple provides temperatures with two decimal places.

The Apple Watch is not available to Android users, so is less accessible an option than some other femtech devices. It's also the most expensive wearable skin temperature tracking device that I investigated, which means it will be less accessible to those with less discretionary spending power

I don’t love wearing things on my hands/wrists at nighttime as I like to have my hands up near my face while I sleep, so I’m not sure whether this option would be suitable for me. I also am hesitant about the EMF levels which seem a little high in comparison with devices like the Tempdrop, Oura or BBRing.

At present, the Apple Watch Series 8 / Apple Watch Ultra doesn’t have much “social proof” for use with FABM charting, and therefore we don’t have much evidence as to how these temperatures might stack up against an oral/vaginal/rectal temperature chart.

So far, I have seen one chart comparing Apple Watch temperatures with oral temperatures, by charter Natalia Rodrìguez in the FAM Support Group Facebook group. The Apple Watch temperatures showed a slightly delayed temperature shift by only one extra day in comparison to oral temperatures; however, there were some missing oral temperatures around the time of ovulation so it is difficult to say how accurate this comparison is.

I think the Apple Watch Series 8 / Apple Watch Ultra may have potential for the future of FABM charting, provided that we begin to see some more evidence or social proof of the correlation between wrist temperatures and oral/vaginal/rectal temperatures. Importantly, there is some evidence that wrist temperatures may delay temperature shifts by around two days, and you can read more about that in the Ava Fertility Tracker bracelet section (again, it is not clear whether Ava studies would be relevant to Apple Watch Series 8, but we are working with the limited data we have at the moment!)

Studies (not an exhaustive list):
Ongoing: The Apple Womens Health Study in partnership with the Harvard School of Public Health. This study utilises data from Apple Watch users (with their consent), to investigate the “relationship between menstrual cycles, health, behavior, and habits.” I think this is a fantastic initiative considering that so much remains unknown when it comes to the intersection of our health and our reproductive hormones as women and/or people who menstruate.

Unfortunately, I cannot find any studies specific to the comparison of Apple Watch wrist temperatures to oral waking temperatures.

Note: I have found that the high levels of radio frequency emitted by the Apple Watch when communicating with my iPhone, interfere with the function of my oral BBT thermometer. I have heard the same for other smartwatches such as the Garmin. I recommend keeping all devices in airplane mode if you are using your BBT thermometer in the morning.

Expand the Ava Fertility Tracking bracelet section to read about some alternative studies on wrist temperatures and menstrual cycle tracking.

Ava Fertility Tracker bracelet

Device: Ava Fertility Tracker bracelet
Price: From USD $280
Type of skin temperatures (proximal or distal): Distal
Skin site: Worn on the left or right wrist (dorsal side) to collect wrist temperatures
Time worn: Worn only while sleeping. Must be worn for at least 4 hours.
EMF exposure: It’s unclear from the Ava website whether the Ava Fertility Tracker emits EMFs while in use, and if so, what the associated SAR level is.
Associated app: Ava Fertility app
3rd Party integrations: None that I am currently aware of
Subscriptions required: No subscriptions required
Type of temperature data: Absolute values
Number of decimals: Two decimal places (.00)
Data points provided:

• Temperature data
• Heart rate
• Heart rate variability
• Breathing rate

My take:
I like Ava as a company because they have always been focused on assisting women and/or people who menstruate with fertility tracking for conception. I prefer this to companies such as Apple or Fitbit who add on menstrual tracking years after introducing their products. It says to me that Ava are wholly focused on us as a user base, as opposed to other companies who see menstrual trackers as simply a subset of their total userbase. I also love that they don’t require a subscription to be able to use their device.

I remain a little sceptical as to whether the Ava device has an acceptable sensor configuration and algorithm to provide accurate enough temperatures for FABM charting. Continue reading for my thoughts on a study that demonstrates this.

As with many of the other Femtech devices covered in this Info Hub, it seems the case that citizen science will need to come to the rescue if we really want to see accurate comparison charts through a FABM lens. Have you used the Ava bracelet and found that it provided accurate temperature readings in comparison to oral temperatures? Let us know in the comments below!

Studies (not an exhaustive list):
Shilaih M, Goodale BM, Falco L, Kübler F, De Clerck V, Leeners B. Modern fertility awareness methods: Wrist wearables capture the changes in temperature associated with the menstrual cycle. Biosci Rep. 2018;38(6):BSR20171279.

Goodale BM, Shilaih M, Falco L, Dammeier F, Hamvas G, Leeners B. Wearable sensors reveal menses-driven changes in physiology and enable prediction of the fertile window: Observational study. J Med Internet Res. 2019;21(4):e13404. doi:10.2196/13404.

Zhu TY, Rothenbühler M, Hamvas G, Hofmann A, Welter J, Kahr M, Kimmich N, Shilaih M, Leeners B. The Accuracy of Wrist Skin Temperature in Detecting Ovulation Compared to Basal Body Temperature: Prospective Comparative Diagnostic Accuracy Study. J Med Internet Res 2021;23(6):e20710.

• Study authors state that a cycle with a positive LH test was considered ovulatory; however, we know that in a small percentage of cases a positive LH test does not guarantee that ovulation will occur. Similarly, a cycle with only negative LH tests was considered anovulatory. Based on this criterion, 11% of cycles were deemed anovulatory.
• Temperature shifts occurred almost two days later with wrist temperatures when compared to oral temperatures.
• Study authors state that wrist temperatures rose to a larger extent (a mean of 11% higher) than oral temperatures in the luteal phase.
• Study authors state that wrist temperatures were more sensitive than oral temperatures, and therefore showed a “higher true-positive rate”; however, wrist temperatures were apparently more prone to showing “false positive” temperature shifts in cycles where no LH surge had been detected.
• My opinion after reading this study is that the Ava bracelet may be unsuitable for FABM charting. This is because temperature shifts will likely be delayed by approximately 2 days. In addition to this, Ava wrist temperatures are more sensitive and rise higher than oral temperatures in the luteal phase, which means that temperature shifts might be confirmed when oral temperatures would not be. Remember, Symptothermal Method rules have been established and studied based on oral/vaginal/rectal temperatures. It’s possible that these existing FABM rules may not be appropriate for the delayed/higher temperature curves created by the Ava bracelet. (Of course, it's also possible that these higher temperature shifts might be beneficial for some charters, especially those who have low progesterone and weak temperature shifts that are not well-served by existing FABM rules.)

Fitbit (various models)

Device: Fitbit devices (certain models/countries only)
Price: From USD $100 (Fitbit Inspire 3 which is a tracker) to USD $300 (Fitbit Sense 2 which is a smartwatch)
Type of skin temperatures (proximal or distal): Distal
Skin site: Worn on the left or right wrist (dorsal side) to collect skin temperatures
Time worn: Fitbit devices are designed to be worn continuously; however, I believe you can choose to wear them at night only if temperatures are all you are interested in.
EMF exposure: I have searched through the Fitbit website and cannot find details of whether Fitbit devices emit EMFs while in use, and if so what the associated SAR levels would be. There are numerous discussions of this topic on Fitbit community support boards, and moderators seem reluctant to disclose actual levels. Fitbit community board moderator "SilviaFitbit" says "The radiation emitted from all Fitbit devices is minimal. It is safe to use your tracker while pregnant.The Fitbit's power output is extremely low: about 0.5% - 1% of the output power of a cell phone. As for our Bluetooth-enabled devices, a good deal of research has been done in the field of Bluetooth radiation dangers to humans, and the general consensus is that the 800MHz to 2 GHz radiation output of a Bluetooth device is negligible".
Associated app: Fitbit app
3rd Party integrations: Fitbit devices are compatible with a wide range of 3rd party apps such as Strava, Peloton, Mindbody, Lumosity, MyFitnessPal and more. Despite this, I am unaware of any 3rd party FABM apps that can harness Fitbit skin temperatures for charting.
Subscriptions required: You can access your average nightly skin temperatures without a premium subscription; however, if you would like to see the full fluctuation of your temperatures through the night, you will need a premium subscription. A premium subscription costs US $9.99 per month.
Type of temperature data: Like the Oura Gen3 Ring and the Apple Watch Series 8/Ultra, Fitbit devices provide skin temperature changes as fluctuations (+/-) from your personal “baseline temperature”. Fitbit calculates your baseline temperature based on up to 30 nights’ worth of previous temperatures. Fluctuation trend data is less useful for FABM charting, as we need actual temperature values to plot on the graph section of our charts each morning. The Fitbit website states that temperature information is not available in all markets.
Number of decimal places: One decimal place (.0)
Data points provided:

• Resting heart rate
• Sleep data
• Oxygen saturation
• Temperatures

My take:
I like that the Fitbit Inspire 3 (at approximately USD $100) is the most affordable wearable skin temperature tracker on this list, meaning it will be accessible to a wider range of people. In addition, a premium subscription is not required to access daily temperature information.

I don’t really like to wear rings or watches to bed, because I keep my hands up by my head while I sleep and I find that jewellery on my fingers/hands interferes with my sleep. That said, the Fitbit Inspire 3 is a much more slimline design in comparison with the Apple watches, which is a drawcard for me.

Unfortunately, just like the Oura Gen3 Ring and the Apple Watch Series 8 / Ultra, the Fitbit app only provides users with fluctuation trends when it comes to daily temperatures. This means that the Fitbit lacks any “social proof” for use with FABM charting because it doesn’t provide actual temperatures which can be used on a chart.

I have personally tested the Fitbit Inspire 3 band and it provided extremely erratic temperatures that were wildly inaccurate on my chart (using the formula further down this page to obtain actual temperatures from the Fitbit fluctuation trends). Due to this, I do not recommend anyone spend money on this device to use for FABM charting.

Studies:
Expand the Ava Fertility Tracking bracelet section to read about some studies on wrist temperatures and menstrual cycle tracking that may have relevance for Fitbit devices.

If you’ve read this far, you’ve now learned how different types of temperatures differ across 24hr and infradian menstrual cycles, and how this might impact FABM charting. You’ve also learned more about the features, pros and cons of some of the main wearable skin temperature trackers available on the market today.

But what about some of the more common frequently asked questions? Expand each of the headlines below to learn more about how to get absolute temperatures, whether your device displays enough decimals for FABM charting, how accurate your sleep data is, how the placebo and nocebo effect might impact you, and more.

Finally, how does using a wearable skin-temperature tracker impact the effectiveness of your chosen Fertility Awareness Based Method? This is arguably the most important question that this entire article addresses, so please take the time to read this section thoroughly.

How can I get an absolute temperature value from my Oura Ring / Fitbit?

Devices such as the Oura Gen3 Ring and Fitbit (various models) do not deliver actual temperature values. Instead, they calculate your “baseline temperature” and then provide you with fluctuation trends as a +/- from this baseline.

With the Oura Gen3 Ring, you do have the option of downloading the Natural Cycles app to be able to see your absolute temperatures; however, a lot of Symptothermal Method charters prefer not to use Natural Cycles app.

Is it too much to hope that these devices might provide absolute temperature values in future? I have read some opinions on the internet that the reason these devices don’t offer absolute temperatures, is that it is too easy for people to compare them against oral/other temperatures and then worry that perhaps the devices are not as accurate as they would have hoped.

Thankfully, FABM charters don’t generally need our temperatures to be perfectly “accurate”. Instead, what we are looking for is precision. We are looking at the overall pattern of our chart to identify temperature shifts. It doesn’t matter whether temperatures are slightly “out” by a certain amount, just so long as that error is consistent across every day of the whole menstrual cycle.

Accuracy: How close a measurement is to the true value.
Precision: How consistent results are when measurements are repeated.

If you have your heart set on using a wearable device like the Oura Gen3 Ring or a Fitbit you will need to find a way to extract absolute temperature values to use on your chart. It's possible that you might be able to do this by assigning a random temperature reading as your “baseline temperature”. As an example, I would assign 36.5°C as a random baseline. Each day, my device will provide me with a figure for how much my temperature has deviated from the official baseline. I would then need to either add or subtract this figure from 36.5°C to provide an absolute temperature value for that day.

For example, if your Oura ring tells you that your temperature is “-0.3°C” you would subtract 0.3 from 36.5 for today’s temperature value of 36.2°C. Alternatively, if your Oura ring tells you that your temperature is “+0.6”, you would add 0.6 to 36.5 for today’s temperature value of 37.1°C.

In this way, you will be able to plot temperature values on your FABM chart. Remember, we don’t need temperatures to be perfectly accurate, we simply need them to be measured consistently each day so that we can identify the temperature shift caused by ovulation.

There are two caveats to this approach.

The first is that it’s possible that your device may update your “personal baseline” temperature without letting you know (in fact, Fitbit directly states on their website that your personal baseline is based on a rolling average calculation of your 30 most recent nights of data). I am imagining that changes to your "personal baseline" could occur for many different reasons over the span of several weeks or months. For example, you stop/start a new thyroid medication (which can impact core temperatures). Alternatively an effect might be seen if you implemented lifestyle changes (for example fasting/increased caloric intake/increased energy output/decreased energy output) that can impact your metabolism and core temperature. I am of the opinion that it is very likely that this could occur, and if so it will impact the precision of all subsequent temperatures on that particular chart.

Secondly, even with manual calculation of daily temperatures based on fluctuations from your personal baseline, Oura and Fitbit Celsius charters may still be lacking a second decimal place. Read on for more information on how the number of decimal places can impact FABM charting.

Finally, remember that you will be operating well outside of “perfect use parameters” if you decide to take this approach to obtain temperatures for your Symptothermal Method chart. Read on to learn more about effectiveness estimates.

Does my device display enough decimal places?

To take your temperatures with the Symptothermal Method, you will need to use a basal body temperature thermometer that is accurate to +/-0.05°C or +/-0.1°F, and displays two decimal places. This is important because the shifts that occur around and after ovulation can be very small, and it’s important that we are able to accurately identify them on our charts.

When it comes to charting your temperatures, you will plot rounded temperatures on the graph section of your chart. Most Symptothermal Methods require that you round your raw temperatures to the nearest 0.05°C/0.1°F (Fahrenheit charters will use scientific rounding, while Celsius charters use a slightly different rounding technique).

Does your device display your daily temperature to two decimal places? It may be that your wearable device has been proven to take highly accurate temperatures in clinical settings, but when it comes to the public app interface, they are only sharing a single decimal place with you.

This specific situation is an issue for Celsius charters, who need to be able to chart to the nearest 0.05°C, which is not possible if your device only displays a single decimal digit.

How might my Body Mass Index (BMI) impact my skin temperatures?

Body Mass Index (BMI) is a number that provides information about a person’s weight and height. It’s useful to note that while BMI does provide some basic information, it also lacks context about things like bone density, muscle mass, overall body composition, and racial differences.

So, always take information based on BMI with a grain of salt, and consider whether the information may/may not apply to you personally.

If you have a higher BMI, I have heard anecdotal stories from other Symptothermal Method charters who have struggled to gain accurate skin temperatures using devices such as the Tempdrop, and believe it may be due to their weight. I’ve heard this from around 3 separate people at this point (not a huge number, but enough to pique my interest!).

Interestingly, there may be some truth to their suspicions regarding BMI and skin temperatures. A 2019 study titled ‘Nocturnal finger skin temperature in menstrual cycle tracking: ambulatory pilot study using a wearable Oura ring’ found that the accuracy of the Oura algorithm increased when the two study subjects with a BMI of over 30 were excluded from the calculations. Study authors state that a BMI of over 30 “is a potential confounder affecting distal skin temperature.”

A 2009 study titled ‘Adiposity and human regional body temperature’ showed that obesity causes distal temperatures to rise, to account for the insulating effect of a greater layer of subcutaneous abdominal fat. These higher distal temperatures allowed study participants to release heat and therefore regulate their core body temperature.

Is it possible that a higher baseline distal temperature might mute the post-ovulatory rise in distal temperatures, and therefore make it difficult to identify an accurate temperature shift? I would love to see more studies on this in future (for both distal and proximal temperatures), and in the meantime it is something that those with a higher BMI may want to keep in mind.

How reliable is sleep data from wearable trackers?

There’s some evidence that sleep data from many wearable devices may not be as accurate as we would like to believe.

Usually, sleep studies are performed under laboratory conditions using an assessment technique known as polysomnography. According to the Mayo Clinic, “polysomnography records your brain waves, the oxygen level in your blood, heart rate and breathing, as well as eye and leg movements during the study.”

Because wearable devices are currently limited to far less data points than are available in clinical sleep studies, it’s no surprise that accuracy may be lacking.

For example, Oura’s own website states that their sleep data only agrees with laboratory polysomnography results 79% of the time!

Additionally, a 2021 study tracked the accuracy of eight separate wearables/associated sleep analysis apps (Apple Watch Series 3, Beddit Sleep Monitor 3.0, Fatigue Science Readiband, Fitbit Ionic, Garmin Vivosmart 4, 2nd generation Oura smart ring, Polar A370, and the WHOOP Strap 2.0). The study found consistent trends across all devices showing failure to accurately determine the amount of time that the wearer was awake as opposed to asleep. Interestingly, the Oura and Fitbit were found to be the most accurate of these studied devices/apps.

These results may be because it is very difficult for wearable devices to differentiate between when you are simply laying quietly in bed with a lowered heart rate trying to fall asleep, versus when you actually are asleep. This can result in overestimation of the total sleep time.

How might inaccurate sleep data affect me?

Have you heard of the placebo effect? The nocebo effect is the opposite of the placebo effect. It describes the situation where someone is more likely to experience a negative outcome if you are expecting (or worried about) a negative outcome.

Have you ever woken up feeling great, but then check your wearable sleep data to see that you barely got any deep sleep the night prior, and you suddenly feel a bit more tired?

Alternatively, do you ever wake up exhausted, but then check your Oura App to see a high “readiness score”, and suddenly pep up a little bit?

I think it’s important to be cognisant of the way that femtech can impact our daily experiences in this way, especially if the sleep data that we are receiving is inaccurate.

There’s a lot to say for simply waking up and listening to what your body is telling you (as opposed to what an app is telling you about your body).

I’m experiencing information overload, will wearables help?

In our fast-paced digital landscape we are bombarded daily by information from all corners of the Internet. A 2012 consumer report by Roger Bohn and James Short of California University found that U.S. consumers were taking in 34GB worth of information each day - a decade later this number is now likely much higher. This is more information than many of us would have been exposed to in an entire lifetime just a few hundred years ago. Now, wearable biometric tracking devices are also contributing to the sheer amount of information we are receiving each and every day. Information overload is well known to contribute to feelings of anxiety, overwhelm, powerlessness and mental fatigue.

If you are someone who struggles with the effects of information overload in our digital landscape, consider opting for a tried-and-true oral basal body temperature thermometer instead. This can be a breath of fresh air for many people. Remember, you know yourself best; if this is the case you can usually find an affordable $20 thermometer from your local pharmacy (and you can find a list of recommended BBT thermometers here).

Not only will you enjoy some respite from the onslaught of digital information, but you will be able to rest easy knowing that oral/vaginal/rectal temperatures are the only ones that have been proven safe and effective for FABM charting in clinical trials.

You’re operating closer to perfect use parameters if you choose a standard BBT thermometer over a wearable skin-temperature tracker.

I’m seriously wanting to prevent pregnancy, are wearable trackers a good idea?

Historically, clinical trials on the contraceptive effectiveness of the Symptothermal Method have assessed basal body temperatures collected from oral, vaginal or rectal sites only.

To meet perfect-use parameters for the Symptothermal Method you need to be relying on oral, vaginal or rectal temperatures. Other temperature collection sites have not been assessed in clinical trials for accuracy when used as a part of the Symptothermal Method.

This means that if you choose to rely on any wearable skin temperature collection device (including the popular Tempdrop), you are now operating outside of perfect use parameters and heading more toward typical use effectiveness rates (although by exactly how much, it is difficult to say).

Depending on how seriously you are needing to prevent a pregnancy, this may or may not be acceptable to you and your partner. This is a situation where you need to be clear on your reproductive intentions and your specific situation.

If it’s important to you to continue using a Symptothermal Method and attain BBT readings closer to your actual core temperatures, consider investigating internal temperature wearables. These include devices such as Ovolane, Trackle, OvuSense and iButton. Many of these are more popular in European markets.

Given that the Tempdrop smart thermometer has a large user base of well over 10,000 users, and has been in use within the FABM community since 2017, many people (myself included) generally have a good level of trust in the accuracy of temperatures provided. I personally use the Tempdrop myself and am satisfied with the charts I receive; it has made postpartum charting a breeze for me. Not everyone shares this good experience though; in fact some charters report that they have struggled to get accurate readings with their Tempdrop. You can read more about the pros and cons of the Tempdrop here. Other wearable skin temperature trackers do not yet share the same social proof for FABM charting that the Tempdrop does.

My recommendation is that if you wish to rely on skin temperatures (from any wearable device!) for use with your chosen FABM, you should spend at least the first three menstrual cycles’ of use comparing your skin temperatures against your oral, vaginal or rectal temperatures. Even after confirming that the temperatures follow the same pattern and you are able to confirm temperature shifts on (or around) the same day, it’s still important to note that you are still operating outside of perfect use parameters. Ensuring you are cross-checking against cervical mucus with a Symptothermal Method likely adds some safety, but is still outside of perfect use parameters. If you're seriously trying to avoid pregnancy, I definitely would not recommend using skin temperatures with a "temperature-only FABM" (such as Natural Cycles).

In saying all of the above, I encourage everyone to experiment … for science! If experimenting with different forms of femtech means you find ways to make your chosen FABM more accessible and convenient – more power to you. Convenience usually means that you are more easily able to stay compliant with the data collection requirements of your FABM, which is beneficial for effectiveness.

So, the bottom line is (as always), you do U-terus. As long as you are making informed decisions based on accurate information, you’re on the right track.

Have you found this Info Hub helpful? Do you have any thoughts to add? Let me know in the comments below; I love hearing what others think about the latest FABM topics.

I would like to preface this article by saying that there is no need to panic if you are a current Daysy user. Instead, this would be a good time to do a little research and re-evaluate whether you are still happy to continue using the device based on how seriously you are wanting to avoid an unplanned pregnancy. Considering we don’t see thousands of women complaining of unintended pregnancy, it is clear that the Daysy device has some level of effectiveness; however, what that effectiveness actually is remains unknown at this point in time.

This week, the study behind Valley Electronics’ claims of a 99.4% effective Daysy device has finally been retracted from the Journal of Reproductive Health. This follows a peer-reviewed scientific commentary by leading reproductive health researcher Dr Chelsea Polis which was published back in June 2018. Dr Polis’ commentary called for the Daysy study to be retracted due to grave methodological flaws which were inflating consumer confidence and potentially putting users at risk of unintended pregnancy. You can read more about the retraction in this Buzzfeed article and more from Dr Polis on her personal blog.

Unfortunately, Valley Electronics (the manufacturer of Daysy) had been aware of Dr Polis’ concerns since 2017 and had made no move to alert their users or amend any of their marketing materials. Following the publication of Dr Polis’ peer-reviewed commentary in June 2018, Daysy again made no further move to alert their customers or amend any of their marketing materials. In fact, their marketing materials have continued to reference a 99.4% effectiveness rating ever since – despite the fact that their study was under investigation to be retracted.

The original peer-reviewed commentary by Dr Polis referenced a number of issues that may have affected the accuracy of the findings of the Daysy study:

1. Users with less than 13 cycles’ use of the Daysy were excluded from the calculations. Within this group of excluded data were 10 pregnancies. We don’t know how this might have affected the effectiveness rates.
2. The survey behind the study only had a 13% response rate. This raised questions about selection bias – would those who had a bad experience with the Daysy device still be in conctact with the company, and if so would they want to participate in such a survey?
3. Daysy relied on users to self-report an unintended pregnancy – they didn’t objectively look at all the data to detect a pregnancy based on extended elevated temperatures.
4. The wording of the survey itself was slightly ambiguous and may have resulted in confusion.
5. The survey didn’t gather information to exclude users that were subfertile (i.e. post-hormonal birth control, postpartum or perimenopausal.

Importantly, Dr Polis’ has emphasised that many of these issues also apply to Daysy’s older studies which used to reference a 99.3% effectiveness rate. Because of this, the true effectiveness of the Daysy device currently remains unknown.

I don’t personally use or recommend temperature-only devices and apps such as Daysy or Natural Cycles, and you can read about why here: 5 Reasons I Don’t Use Daysy Or Natural Cycles. 

Essentially though, the main reason is that basal body temperature is not a predictive fertility biomarker – it only rises after ovulation has already occurred. Because sperm can survive up to 5-7 days in the reproductive tract in ideal conditions, we need to have advance warning that ovulation is approaching. Basal body temperature cannot provide this advance warning, as it only rises after ovulation has already occured. This means that Daysy and Natural Cycles are making educated guesses about when you might be fertile in your current cycle, based on data from your past cycles and other users. This is problematic because there are many external factors that can cause you to ovulate earlier or later than expected (such as stress, illness, travel, supplements, diet and lifestyle changes).

I recommend the Symptothermal Method of Fertility Awareness instead, which tracks both cervical fluid and basal body temperature. Cervical fluid is influenced by oestrogen levels, which rise in the lead-up to ovulation. In this way, cervical fluid can provide advance warning of ovulation, whereas basal body temperature cannot. If you are interested in learning the Symptothermal Method I recommend working with an instructor and you can find one here.

If you’re seriously needing to avoid pregnancy, it’s important to work with an instructor of the Symptothermal Double-Check Method. Why? Because as it stands, no studies have investigated the effectiveness of self-teaching the Symptothermal Method: the effectiveness of self-teaching currently remains unknown.

If you’re a big fan of femtech in your day-to-day life, there’s no need to resort to charting your cycles on paper if that’s not a good fit with your lifestyle. There are a number of free apps such as Kindara that you can use to record your charting observations, and the Tempdrop is a great option if you’re after a wearable thermometer that takes your temperature throughout the night. The key here is that this femtech is used to collect and chart your information – interpretation of that information remains up to you, the user.

Anecdotally, if you are absolutely certain that you wish to use either Natural Cycles or Daysy, it appears that Daysy has a much safer algorithm than Natural Cycles. The amount of unintended pregnancies and scares that I see with Natural Cycles users is truly concerning. If you are seriously trying to avoid pregnancy please steer very clear.

Ultimately, I believe both options are simply glorified and expensive thermometers dressed up in high-tech disguise; however, they can be useful tools for those who have no desire to learn to chart their cervical mucus – as long as those users are fully informed of the accurate effectiveness rates. In saying this, the real benefits of Fertility Awareness-Based Methods lie in the body literacy they provide – and Daysy and Natural Cycles come nowhere close.

Have you used the Daysy device or the Natural Cycles app? I would love to hear your experience in the comments below.

Daysy and Natural Cycles are right for some people, but they are not right for all people. To know whether these products are the right choice, customers really need to have been provided with adequate information through the education and healthcare systems as to how their fertility actually functions. Unfortunately, fertility awareness is barely covered by schools and GP’s, leaving a large majority of women with a substantial knowledge gap when it comes to their menstrual cycles. This means that companies existing within the femtech space are largely marketing to an uninformed audience who cannot critically evaluate the claims that are made about effectiveness.

In many ways, this is not the fault of companies such as Daysy/Natural Cycles, but the fault of the social structures in which they exist.

The below video is my personal opinion as to why I choose NOT to use these apps/devices.

Press play to join the conversation.

For those of you who prefer to read, I’ve included a brief summary below of the points I make in the video.

1.) Natural Cycles and Daysy/Ladycomp are temperature-only methoDS:

Your basal body temperature generally only shows a sustained rise *after* you’ve ovulated (due to the release of progesterone by the corpus luteum after ovulation). This means that your temperature does not give you any real-time information about the state of your fertility prior to ovulation (apart from telling you that you haven’t ovulated yet).  If you’re trying to avoid becoming pregnant, you need to know at least six days prior to ovulation, that ovulation is approaching. This is due to the fact that sperm can survive for up to 5-7 days in the reproductive tract. Cervical fluid is your warning sign of approaching ovulation, due to the fact that cervical fluid is influenced by rising levels of oestrogen. Natural Cycles and Daysy/Ladycomp do not take cervical fluid into account. Instead, they predict your current cycle based on past cycles. This can be risky for those who experience an unexpectedly early ovulation.

2.) Temperature-only methods were superceded by the Symptothermal Method in the 1950s:

Temperature-only methods were used in the 1930s, before it was discovered in the 1950s that cervical fluid allowed a user to “see” oestrogen levels rising in real-time. For an in-depth overview of the transition and development of Fertility Awareness-based Methods see this post here:

3.) Effectiveness and research:

As of the date of publishing this article, the typical-use probability of an unintended pregnancy after 13 cycles of use are as follows:

Natural Cycles: 8.3%
Daysy: 2.7%
Symptothermal Method(Sensiplan): 1.8%

To put this into perspective: If 5000 women are using each of the above contraceptive methods, after 1 year of use 415 Natural Cycles users will experience an unintended pregnancy, 135 Daysy users will experience an unintended pregnancy, and 90 Sensiplan users will experience an unintended pregnancy*. This is based on “typical use”  – so users who are making mistakes and/or being careless.

*Note: Because these percentages have been taken from a range of different types of studies (including clinical trials and retrospective surveys), it is difficult for a comparison to be truly accurate – these figures are just to give you a rough idea.

Natural Cycles’ latest study was questioned by the Natural Fertility Department of the German Society of Gynecological Endocrinology and Reproductive Medicine. You can read their concerns here: https://sektion-natuerliche-fertilitaet.de/warnung-vor-fda-zugelassener-verhuetungs-app/?fbclid=IwAR0YvpBQV7-HRcIW3ILH0zkhGyFPlRhO-ndjEzXXcaa0nWbw9-QMpWJS7R4

Daysy’s latest study was questioned by Chelsea Polis, a senior researcher with the Guttmacher Institute, and you can read about her concerns here: http://chelseapolis.com/1/post/2018/06/pushing-daysys-how-people-could-be-misled-into-buying-an-unproven-device-for-contraception.html

As a comparison, you can read about concerns that were raised about Symptothermal Method studies here: https://fertilitycharting.com.au/2018/08/10/new-research-released-on-the-effectiveness-of-fertility-awareness-based-methods/

4.) Marketing:

Despite the concerns raised about the issues (both minor and major) with the Daysy and Natural Cycles studies, both companies continue to aggressively market their products to a primarily young demographic on social media. They often do this through the use of social media influencers who either happily receive a commission by referring their followers to purchase, or are paid to create a sponsored post. Natural Cycles especially concern me given the fact that their founder Elina Berglund is quoted as saying that the ideal user “is a woman in a stable relationship who is planning to have children at some point, and who would like a break from hormonal contraception ahead of trying.” Kristina Gemzell Danielsson is a professor in obstetrics and gynaecology at Karolinska Institutet and is one of the research scientists behind the Natural Cycles studies. She is quoted as saying that Natural Cycles is “not a good option for women who absolutely want to avoid a pregnancy”.

5.) body literacy:

Placing your fertility in the hands of an app or device is in some ways just another way of handing over your power and autonomy. What happens when you lose your phone or your batteries run flat? Without the deeper level of knowledge about how our fertility functions, we are stripping women of the chance to experience true empowerment and reproductive freedom. When Daysy and Natural Cycles only give users half the picture (temp-only as opposed to informing users of the importance of cervical fluid), they are simply keeping them tethered to an expensive product that provides a sense of security (however misplaced), at the expense of true body literacy.

conclusion

If Daysy and Natural Cycles are the right choice for you, you will know. Each person has the freedom to make their own choices when it comes to contraception and for some people, a temperature-only method will be an ideal fit for their lifestyle and their reproductive intentions. My hope is that the above video will help you to understand the full picture, so that you can provide truly informed consent about the Fertility Awareness-Based Method that is right for you.

Want to start using the Symptothermal Method of Fertility Awareness? Consider working with an instructor, or read about self-teaching here.

You may have heard that Natural Cycles was given the green light by the FDA last week to market itself as an official contraceptive. Natural Cycles predicts when a user can/cannot become pregnant based on that user inputting their daily basal body temperature reading.

Understandably, the internet has been abuzz with proponents and sceptics alike debating the wisdom of approving such an app, while women in their hundreds have been chiming in with anecdotal experiences, both negative and positive.

However, after almost two years of tracking my own fertility to avoid pregnancy with the Symptothermal Method (cervical fluid and basal body temperature), you won’t find me recommending women to use an app such as Natural Cycles as their sole form of contraception.

Curious as to why? Below you’ll find the seven key reasons that I don’t personally recommend Natural Cycles to the women in my life.

1.) Using your basal body temperature (BBT) fluctuations to track when you are/are not fertile is not new.

It was first discovered by Dr William Squires in 1868 in a London tuberculosis sanatorium that the BBT of fertile women seemed to rise in the second half of their menstrual cycle. This was also noticed around the same period by Dr Mary Putnam Jacobi in Philadelphia; however, neither Squires nor Jacobi realised that this biphasic pattern was related to ovulation. It wasn’t until 1904 that Dutch gynaecologist Theodoor Hendrik van de Velde in Holland discovered that the rise was related to the event of ovulation. In 1928 he confirmed that it was due to the thermogenic effect of the hormone progesterone which is secreted by the corpus luteum after ovulation.

Also during the 1920s, Japanese gynaecologist Dr. Kyusako Ogino and Austrian obstetrician-gynaecologist Dr Herrmann Knaus were separately conducting research on ovulation and the timing of fertile days during the menstrual cycle. The Ogino-Knaus system was the precursor for the calendar rhythm method, which was publicised to American women in 1932 by Dr Leo Latz with the publication of his book The Rhythm of Sterility and Fertility in Women. Unfortunately, many of Latz’ fellow doctors were unhappy with this information being made available to the public and he was dismissed from the faculty of the Loyola University School of Medicine.

Fast-forward to 1935 and the research on both BBT and calendar/rhythm was eventually harnessed by a German Catholic priest called Father Wilhelm Hillebrand who was trying to help some of his parishioners who had serious health reasons to avoid a pregnancy. He at first advised his parishioners to use the Ogino-Knaus calendar/rhythm system; however, when three of the women fell pregnant almost immediately, Father Wilhelm Hillebrand went in search of better options and came across van de Velde’s research on BBT. He decided to combine BBT observations with the Ogino-Knaus calendar calculations and the Calculo-Thermal method was born.

Then, in the late 1940’s and 50’s, the cyclical changes in cervical mucus and their relationship to ovulation started to receive more attention. While cervical mucus had first been described in 1855 by London doctor W. Tyler Smith, nobody thought to link the changes in mucus to cyclic changes in fertility – cervical mucus was then promptly forgotten about for the next 100 years! However, this all changed in the late 1940s and 50s when a number of scientists revisited the role of cervical mucus in fertility (Erik Odeblad and John and Evelyn Billings being some of the more well-known). Around the same time, in 1953 New York obstetrician-gynaecologist Dr Edward F Keefe added daily mucus observations to the Calculo-Thermal method, and the Symptothermal Method was born. The Symptothermal Method was then further developed and promoted by Dr. Konald A Prem in America, and Dr Jozef Roetzer in Austria.

PHEW. This point turned into an essay but the gist of it is this: the temperature method of fertility awareness has been around since 1935 and was superseded by the more reliable Symptothermal Method (Cervical fluid + BBT) in 1953.

2.) Natural Cycles doesn’t appear to be a very conservative temperature-only method of Fertility Awareness.

One of the pioneers of the temperature-only method of Fertility Awareness was a German doctor called Gerhard K. Doering who released research in 1967 that highlighted the importance of

1. Waiting until the evening of the third raised temperature, due to the fact that he documented a number of pregnancies where conception occurred from intercourse on the second raised temperature. This was also to ensure that the rise was not due to a fever or disturbance.
2. Using a rule (now known as the Doering Rule) to calculate the last “infertile” day after menstruation, based on the earliest thermal shift recorded during the last 12 consecutive cycles.

Unfortunately, I have seen a number of Natural Cycles charts where green days are given on the second raised temp, and green days are given that extend beyond where a Doering rule would have cut them off. This places the unsuspecting user at risk of unintended pregnancy (and unfortunately I have seen some of those pregnancy charts, too).

3.) Basal body temperature only rises AFTER you’ve ovulated. Taking your temperature every morning will not warn you that ovulation is approaching – this is especially dangerous if you experience an ovulation that occurs unexpectedly earlier than usual.

Cervical fluid is more important than BBT in the follicular phase because it is the only physiological sign (apart from cervical changes) that is affected by oestrogen. Oestrogen increases as ovulation approaches. In short, Natural Cycles cannot predict or protect you from an early ovulation, and nowhere have I seen them mention this in any marketing materials. You would need to be checking your cervical fluid every day (and using some form of Doering rule) to safeguard against an unexpectedly early ovulation.

4.) Natural Cycles’ marketing materials don’t seem to mention that their app is not suitable for women WHO WOULD BE DEVASTATED BY AN UNINTENDED PREGNANCY.

If you visit the practitioner section of the Natural Cycles website, a flowchart advises that GP’s should direct these women to choose an IUD instead. Elina Berglund, the original creator of Natural Cycles and previous CERN physicist says that the ideal user “is a woman in a stable relationship who is planning to have children at some point, and who would like a break from hormonal contraception ahead of trying.” Kristina Gemzell Danielsson is a professor in obstetrics and gynaecology at Karolinska Institutet and is one of the research scientists behind the Natural Cycles studies. She is quoted as saying that Natural Cycles is “not a good option for women who absolutely want to avoid a pregnancy”.

5.) Natural Cycles SAYS that they are more effective than other Fertility Awareness-Based Methods.

The abstract of the most recent Natural Cycles study states “This study shows that the efficacy of a contraceptive mobile application is higher than usually reported for traditional fertility awareness-based methods”. I guess this really comes down to semantics. What do you class as a “traditional” method of Fertility Awareness? I can certainly understand how Natural Cycles should be more effective than the Rhythm Method (calendar only), however that’s not exactly a high standard to beat! Moreover, it has been proven that using a Symptothermal Double-Check Method (cervical fluid, BBT and a calendar-based rule such as the Doering rule) is more effective than a single-check method (temp-only or cervical mucus-only). I have also seen on a number of occasions Natural Cycles quoting statistics from the CDC to ensure that their own statistics look incredibly effective in comparison to other FABM’s. The CDC unfortunately are required to lump together all FABM methods when calculating typical-use efficacy rates – this means that the high efficacy rate of the Symptothermal Method is obscured by the inclusion of the Rhythm method. You can read more about that on the FACTS website here: https://www.factsaboutfertility.org/petition-the-cdc-women-and-medical-professionals-need-accurate-information/

6.) Just because the FDA approved Natural Cycles, doesn’t mean I’LL BE leaping off MY seat to purchase a subscription.

Let’s not forget that the FDA also approved controversial permanent contraceptive implant Essure, which is now the subject of class action lawsuits and over 27,000 complaints of injury (many serious and irreversible). I’ll be taking this approval with a grain of salt.

7.) Natural Cycles only gives their users half of the picture.

When Natural Cycles uses language like “the app does the work for you and tells you exactly when to use protection, so you can learn about your body and cycle along the way” it is a contradiction in and of itself. If the app is doing all the work for the user, the user does not have the chance to develop any level of body literacy above a basic understanding that an ovulatory cycle should have a visibly biphasic temperature pattern. Additionally, Natural Cycles doesn’t mention crucial information about the importance of cervical fluid observations to avoiding (or gaining!) a pregnancy. In my book, this does not qualify as “getting to know your cycle.”

While I would not recommend Natural Cycles, I can also understand the relevance of the saying “A less-effective contraceptive used well, can sometimes be more effective than a highly-effective contraceptive used badly.”

For some women out there who are not inclined to use the Symptothermal Method carefully and correctly, Natural Cycles might be more effective. For some women with extremely regular cycles, Natural Cycles will probably work very well and may never present an issue. For these women, Natural Cycles can be a good gateway into discovering how Fertility Awareness works with your body. Alternatively, some women might simply like the user-interface of the Natural Cycles app, and may prefer to use it in conjunction with their existing Symptothermal Method knowledge.

However, I remain concerned about the women who will stumble across the Natural Cycles app and blindly place their trust in its predictions, without realising its potential shortcomings. If and when a percentage (however small) of these women experience an unintended pregnancy, it will more often than not reflect badly on the entire Fertility Awareness community – which is a shame considering the delicate journey to gain more mainstream awareness and use of modern FABMs.

Have you used Natural Cycles? I would love to hear your experiences (both good and bad!) below.

Fitbit (the popular wearable technology company that allows users to track metrics such as activity, exercise, sleep, weight and more), has finally released their long awaited ‘Female Health Tracking’ feature this week. (And if you weren’t aware of this, well I don’t blame you for living under a rock – things are probably nice and peaceful under there).

However, the Interwebs have been abuzz with hype – from Coachmag, TechCrunch and VentureBeat, to Lifehacker, YourStory and Gizmodo. Basically, the new technology allows users to input data on the dates and details of their menstrual bleeding, along with basic cervical fluid observations. Fitbit then uses this information to predict the dates of your upcoming fertile window and your next period.

Wow. Sounds useful, huh?

To a point, yes. But I’ll argue that these sorts of predictions don’t really tell you a lot about your body with any certainty, and can actually cause more harm than good when users aren’t fully educated about the intricacies of the menstrual cycle.

In fact, this blog might have been more aptly titled ‘Why You Should Never Trust Period App Predictions Full Stop!’

First things first. You can’t predict future cycles based on past cycles. Sure, you might see some pretty consistent patterns if you happen to be tracking aspects of your menstrual cycles such as:

• calendar dates
• colour, amount and consistency of menses (period blood)
• basal body temperatures
• cervical fluid observations
• cervix height, openness and softness, and
• lubricative vaginal sensations

However, there are many factors that can temporarily impact your menstrual cycle. These include diet, exercise, travel, stress, illness and more – and it’s not realistic to expect that Fitbit will be able to account for these temporary lifestyle factors when predicting your next fertile window or menstruation. For example, a stressful life event could cause you to ovulate far later than usual, or certain supplements and dietary changes could even cause you to ovulate much earlier than usual.

Secondly, (and this point is slightly less relevant for users who will be inputting cervical fluid observations): Just because you’re inputting the start and end dates of each period, doesn’t mean an App can accurately predict your fertile window. Why exactly is this? Well, it boils down to the fact that most apps will assume you are ovulating approximately 14 days prior to the first day of your period. This 14 day timeframe is known as the luteal phase and is marked by a rise in Progesterone levels released by the Corpus Luteum after you have ovulated.

The problem with the assumption of a 14 day luteal phase is this: not everyone has a healthy luteal phase. You could have a textbook 28 day menstrual cycle, and an App would predict ovulation around Cycle Day 14. However, in reality you could be experiencing delayed ovulation followed by a short 7 day luteal phase. This would be easier to spot if you were tracking cervical fluid and basal body temperature, but for those users inputting dates only, Fitbit (or any app for that matter) is very unlikely to correctly predict your fertile window in this instance.

The above scenarios can all cause issues for people who rely on app predictions when trying to avoid an unplanned pregnancy or when trying to conceive a child. And the word ‘issues’ is really an understatement here when you consider the stress and stigma associated with an unplanned pregnancy, and the heartbreak associated with struggling to conceive a child.

The good news is that using a method of Fertility Awareness completely eliminates the need for predictions of any sort. Why? Because every single day, you ask yourself the question ‘Am I fertile TODAY?’ Depending on the method of Fertility Awareness that you use, your answer will be based on a combination of cervical mucus observations, basal body temperature readings, hormone level tests and more. What your answer won’t be based on is app predictions generated from previous cycle dates.

So in short, Fitbit’s Female Health Tracking feature looks like it will offer you a nice way to record some basic observations about your menstrual cycle, but if you’re serious about trying to conceive (or avoiding a pregnancy), you won’t be wanting to implicitly trust their predictions. (Or the predictions of any other period tracker app).

In fact, you’re better advised to invest your hard earned dollars in securing the services of a qualified Fertility Awareness Educator (find one here!) to teach you how to interpret your own biomarkers of fertility. You can then use this information to either avoid or achieve pregnancy (and that’s a whole other story altogether).

I’d love to hear whether you’ve taken a look at the new Fitbit Female Health Tracking feature – please share your thoughts below!