Indeed, given the legal complications of reproductive health in some states, privacy would be at the top of my mind if considering to use an app like this.
A few years ago, I wouldn't have been very bothered by the privacy implications of an application like this, but for obvious reasons it's become much more prudent to consider them. Were I a woman I'd be deeply troubled to use an application like this, no matter how good, without some kind of ironclad proof of a privacy respecting codebase. Imagine logging patterns indicative of a pregnancy followed by an abortion, and then imagine selling this data to Texan bounty hunters[0] or any other nefarious actor in your jurisdiction. The stakes have become too unacceptably high to use an application like this without knowing with perfect certainty that it is safe to use.
Which isn't to cast any aspersions on OP for developing it. But the game has suddenly become very dangerous, and despite their reassurances, this is in fact a weapon of potentially lethal consequence.