Comment by mrweasel

> Use a paper calendar.

Well yes and no. I do see what you're getting at, but in certain environments, e.g. abusive relationships, it might be easier to keep you secrets on your phone. You're more likely to be able to keep track of it at all times, as compared to a notebooks or a calendar. It can also store years of data easily. Ideally storing these things in the cloud would be safer that storing them on device or on paper, but that assumes that the cloud providers can be trusted.

So in this case the menstrual tracking app would be a cloud service that only you have access to. Data stored encrypted, only to be unlocked when you access it. It's just, you can't monetize that easily and how would people who truly need 100% privacy pay? Bitcoin, how would they get Bitcoins?