https://www.techradar.com/news/apple-is-adding-menstrual-cycle-tracking-to-watchos-6

Apple has announced that its new watchOS 6 will come with Cycle Tracking, enabling users who menstruate to keep an eye on their monthly cycle. 

The feature will allow users to log key aspects of their periods and fertility, even sending users notifications when their period is about to start, giving them a heads up to stock up on sanitary supplies (and painkillers).