- When used with Apple Watch, Natural Cycles is 93% effective with typical use and 98% effective with perfect use, i.e. the same as if used with a basal thermometer or an Oura Ring
- This was shown in the clinical evaluation Natural Cycles conducted and then reviewed by the FDA. It’s important to note that using Natural Cycles perfectly means avoiding unprotected sex on days the app has identified as fertile (Red Days)
BBT and wrist temperature
When taking your oral temperature first thing in the morning, you are measuring your basal body temperature (BBT), which is the lowest value of the core body temperature (CBT) that happens when you sleep. It is well known that BBT is well correlated with ovulation, being lower in the follicular phase, and higher after ovulation has occurred. These established temperature patterns is how the Natural Cycles algorithm is able to determine if you have ovulated or not based on your temperature data.
When worn at night, an Apple Watch that has temperature sensors (Series 8 and higher), measures your wrist temperature. This measurement is related to the body temperature and can also be used to detect ovulation. While you sleep, your Apple Watch samples your temperature every five seconds.
To improve accuracy and reduce bias from the outside environment, the Apple Watch has two temperature sensors—one on the back crystal, near your skin, and another just under the display. The data collected during the night is processed using Apple’s advanced algorithms that provide an aggregate temperature value for each night. This temperature value can be used by the Natural Cycles fertility algorithm to determine if you have ovulated.
It’s important to note that while BBT and wrist temperature can both be used to detect ovulation, they are two distinct physical quantities and should not be compared directly. Just like fever measurements when taken orally, rectally, in the armpit, in the ear, or on the forehead all will be different.
For this reason, the Natural Cycles algorithm will never mix temperatures taken orally with those taken from an Apple Watch, but will instead establish a new coverline for users switching from an oral thermometer to using an Apple Watch. This also means that you might experience some more Red Days than usual in the first cycle after switching devices, since the algorithm now has less temperature data to go on at first and always errs on the side of caution.
Using the NC° app with Apple Watch: clinical evaluation results
Immediately following Apple’s announcement that they would be adding temperature sensors to Apple Watch, Natural Cycles users began asking for this integration. This quickly became our number one user request and our Research Team began validating the quality of the wrist temperature data, just as we had done with the Oura Ring.
At Natural Cycles we’re grounded in science, so of course the first step was to design a clinical evaluation to establish if Apple Watch measurements were sufficiently accurate to be used in combination with the Natural Cycles algorithm. It was important to learn if the device could deliver comparable performance as the oral thermometer measuring with two decimals.
Our research did not intend to change the way the Natural Cycles algorithm detects ovulation or calculates the daily fertility status, but rather test to see what happens when we replace oral measurements with wrist measurements from Apple Watch.
We collected data from 272 women and a total of 505 complete menstrual cycles. The participating women wore a compatible Apple Watch nightly, as well as measured their temperature with their usual oral thermometer. Participants also collected LH test results, which we used to have an independent reference for ovulation detection.
The results of the clinical evaluation demonstrated that when temperature was inputted from either the Apple Watch or the oral thermometer, the Natural Cycles algorithm was able to identify that ovulation had occurred. The difference in the number of Green Days provided by the Natural Cycles algorithm when the temperature data was provided by Apple Watch (compared to the two-decimal place oral thermometer) was less than 2% or 0.6 days per cycle.
The FDA evaluated our clinical evaluation alongside other software and cybersecurity information that we provided, and cleared the Apple Watch integration - deeming it safe & effective. The results are included in the Summary of Clinical Performance Testing section of our 510(k), that you can find here.
Natural Cycles also received clearance to integrate with Apple Watch in Europe and has been registered in Australia. This means that all women above 18 in the United States, Europe, and Australia can now use our app powered by Apple Watch for contraception.
While we plan on getting the proper regulatory clearances in additional markets where Natural Cycles is able to be used as a contraceptive device with the oral thermometer, such as South Korea, and Singapore, for now in these markets Natural Cycles can only be used with Apple Watch for fertility monitoring.
How to use the NC° app with your Apple Watch
As with any other measuring device, be it an oral thermometer or the Oura Ring, how the device is used or worn matters to ensure the best data quality. In the case of Apple Watch, this means wearing the device with the right fit — preferably on your non-dominant hand and not too tight or not too loose – with room for your skin to breathe.
Choosing the right band material is also important, to ensure that you feel comfortable wearing the watch at night. Finally, some users find it more comfortable to wear the watch on the inner wrist instead of the outer wrist. This is fine, as the increased blood flow in the inner wrist can actually lead to more stable temperatures.
Once you’ve figured out your preferred way of wearing your Apple Watch at night, just enjoy your sleep, and we will do the rest.
Eleonora Behnar, Head of Science and Data