When Apple announced the skin temperature sensor in the Apple Watch 8 and Apple Watch Ultra, it made headlines that the watches would be used to track fertility.

The Apple Watch Series 8 and Apple Watch Ultra use two built-in sensors to take skin temperature readings every five seconds throughout the night (these sensors will also be used in the Apple Watch Series 9 and Apple Watch Ultra 2 (This sensor will also be included in the Apple Watch Series 9 and Apple Watch Ultra 2, which will be available this month).

At the time, Apple said this would help the built-in menstrual cycle tracking app give users an idea of when they are ovulating, since body temperature rises in response to ovulation. This is helpful for both women who want to get pregnant and those who want to avoid pregnancy. However, it is not perfect, and using the Apple Health app to predict ovulation was a bit tricky.

Starting today, however, the Apple Watch can be integrated with the fertility app Natural Cycles. With this integration, the Natural Cycles app can use skin temperature readings from the Apple Watch to predict fertile and non-fertile days to help women prevent and plan for pregnancy; instead of taking individual measurements with a thermometer, as is the case with the Oura Ring integration, the app reads data from the Apple Watch each morning and updates its status.

Natural Cycles is an FDA-approved contraceptive app; those who wish to integrate their Apple Watch with Natural Cycles must purchase a Natural Cycles subscription and sync their device through the Natural Cycles app. Natural Cycles costs $119/69/year or $9.99/9.99/month.

Since wrist temperature is measured during sleep, the device must be in sleep mode in order to measure body temperature on the Apple Watch. If the device is not in sleep mode, the wrist temperature will not be measured.

Body temperature naturally fluctuates and changes nightly due to diet, exercise, alcohol consumption, sleep environment, or physiological factors such as menstrual cycles or illness; Apple Watch will determine a baseline for your wrist temperature after about five nights of tracking and examine the changes nightly. If you upgrade to a new Apple Watch this month, you will need an additional five days to re-establish the baseline data.

To view your temperature data on the Apple Watch, access the Health app on your iPhone. From here, tap Browse, then Body Measurements, then Wrist Temperature. Most women experience a slight rise in body temperature after ovulation, which remains high if pregnant, and drops again before the next period if not pregnant.

When using Apple Watch temperature data to predict fertile days, it is important to wear it consistently at night, make sure it is in sleep mode, and make sure it is tight enough at night. If the device is worn too loosely on the wrist, the readings are more likely to be inaccurate.