I used these kinds of apps when I was a teenager. I could not keep up with a calendar - I tried but my cycles were too irregular to be predictable based on the calculations I could find in books and the internet. I’m transgender and found the entire experience unpleasant in a fairly intense way.

Those kinds of apps helped me immensely. Most of them offer some sort of discreet icon or password system - my parents were the type to read my diary/calendars. My periods are all over the place, and I was able to safely log a pattern in related pain/duration/quantity… most of them included links to places to find medical information. I found one that wasn’t pastel pink and just treated me like a person keeping track of their medical statistics. I got to feel neutral about a part of my body which I despised.

The problem is not that people are naive enough to hand over the information to a third party they can not trust. The problem is the paucity of information and resources for menstrual health. Periods are complicated and scary. When you’re a teenager, you’re not worried about data security - you’re worried about trying to make sure you know when your next cycle will start so that you don’t experience the hell that is bleeding through your pants in math class.