Given the current restrictions in the U.S., Kendra Albert, Maggie Delano, and Emma Weil discuss data privacy for those who track their periods:

In their investigation, police try to find evidence that someone intended to miscarry, or was otherwise endangering the viability of their pregnancy. This is because a medical abortion presents the same way as a miscarriage, and prosecutors must prove intent or willful endangerment of an embryo or fetus in order to convict someone (though being arrested at all is traumatizing and can cause severe health consequences). Prosecutors must be able to prove their case beyond a reasonable doubt — data from a period tracker app is not enough on its own to prove this, even if it’s relevant.

I think there’s understandably been nervousness around tracking your period, but it seems that from a legal perspective, there’s little risk? Albert, Delano, and Weil also recommend privacy-centric apps and discuss the more technical aspects in a companion article.