You’ll soon be able to log your menstrual health in the Fitbit app
https://www.techradar.com/news/youll-soon-be-able-to-log-your-menstrual-health-in-the-fitbit-app
Fitbit is set to update its app later this year to allow women to track their menstrual health alongside the long-standing stats for your steps, heart rate, fitness and sleep.
You'll be able to log details relating to your menstrual cycle which will work alongside the data Fitbit normally provides to get a more complete snapshot of your overall health.