What is menstrual health tracking in the Fitbit app

The menstrual cycle is a recurring cycle in which the lining of the uterus thickens to prepare for the possibility of pregnancy and then sheds if pregnancy doesn't occur (source). The shedding of the uterine lining is the bleeding phase of the cycle, also known as the period (source).

Your menstrual cycle starts on the first day of your period and ends the day before your next period (source).

The predictions for your period, fertile window, and cycle phases are estimated with an algorithm that includes the period and cycle length information you provide. It’s also affected by ovulation test results that you log.

If you don’t provide some of the information about your cycle, Fitbit uses a 28-day cycle and 5-day period as a starting point for estimations. Your fertile window is always shown as 7 days in the Fitbit app, which includes the 5 days before ovulation, the day of ovulation, and the day after ovulation.

Period and fertile window predictions become more personalized as you log your period consistently.

You can edit the average cycle length and period length you provided during setup. However, these settings only affect your initial predictions. For more information, refer to How do I use the Fitbit app to track my period?