Use Cycle Tracking on Apple Watch
Use the Cycle Tracking app 
to log details about your menstrual cycle. You can add flow information and record symptoms such as headaches or cramps. Using information you’ve logged, the Cycle Tracking app can alert you when it predicts that your next period or fertile window is about to start. For more information, see the iPhone User Guide and the Apple Support article Track your period with Cycle Tracking.
Set up Cycle Tracking
Open the Health app on your iPhone with iOS 14.
Tap Browse at the lower right to display the Health Categories screen.
Tap Cycle Tracking.
Tap Get Started, then follow the onscreen instructions to set notifications and other options.
Scroll to the bottom of the page, tap Options, then turn on the options you want—Period Prediction and Fertility Prediction, for example.
Log your cycle on Apple Watch
Open the Cycle Tracking app
on your Apple Watch.Tap the buttons and choose options that describe your period—your flow level and symptoms, for example.
The observations you provide appear in the Cycle log on iPhone. If you’ve turned on Period Prediction and Fertility Prediction in the Health app on iPhone, you will receive notifications on Apple Watch about upcoming periods and fertility windows.
On iPhone, you can also log factors in the Health app that could impact your cycle, such as pregnancy, lactation, and contraceptive use. Depending on the factors you select, your period or fertile window predictions on iPhone and Apple Watch may be turned off. For more information, see the iPhone User Guide.
Important: Fertile window predictions should not be used as a form of birth control.