> If you have a regular period, why do you need an app at all?

You probably don't need to use it if your cycle is completely regular and it doesn't really impact your daily life, but it's not as common as you might think: about 10% of women have PCOS, which is the leading cause of oligomenorrhea; about 10% have endometriosis, which often causes debilitating pain and irregular periods (with a small overlap with PCOS population); 20% to 30% live with PMS - and that's only the portion that has clinically significant symptoms. Even if you were lucky enough to avoid all of these, your cycle length will change as you age, gain or lose weight, and inevitably reach menopause.

Still, you'll have to at least mark the dates. Someone here in the comments compared it to tracking completely optional fitness metrics like sleep or steps, but period data is not really in the same bucket. Just as an illustration: it's hard to see a doctor without being asked "when was your last period?" or "any chance you might be pregnant?", no matter what brought you into the office. In fact, it is such a common experience that it became a subject of many jokes [1]. Also, if you only rely on your memory, you might not notice if/when you do experience changes, some of which might be medically significant.

But let's say you've already decided to track your data somehow.

> what does the app give [...] does it do anything you can't do with a simple notebook app?

Valid question. Some people do just use notes, especially when they don't experience any problems and don't care much about when their next period is coming. But for many others, there are plenty of valid use cases:

1. Reminders for ovulation and next periods. The app can also remind you to enter the data if it thinks you should've had a period but you didn't enter anything. 2. Sharing with your partner. You could, theoretically, write it in a shared document or hand over your paper notebook in person, but it's much easier to see this type of data in a calendar rather than do mental math every time. Having this option gets even more important if you are trying to conceive and track fertility windows. 3. Not having to do the aforementioned mental math is also convenient for the woman herself. A lot of women, even completely healthy ones, experience an array of various unpleasant symptoms in the luteal phase, as well as changes in mood, physical and even cognitive performance during the cycle. It's just really useful to be able to quickly see the calendar and have an idea of what to expect while making your plans (for example, people might want to adjust their workout routines, book a vacation on a more convenient date, or avoid taking extra responsibilities when they know they are going to feel shitty).

And now for those who were not as lucky.

> If you have an irregular period, does this app help "guess" when it's going to start/end?

It does! Though surprisingly, a lot of apps, including Flo, are still abysmally bad at this: they either give you a median of past cycles, at best unhelpfully telling you that your periods are "late," or require you to enter lots of sensitive and subjective data daily to get useful predictions. It is well-known in medical literature that there are other metrics like resting heart rate and skin temperature that are predictive of different phases, especially when they are combined with other data. I've always wondered why the integration with consumer wearables that track a lot of those indicators with good-enough precision is not commonplace. As far as I know, only Apple Health's cycle tracking feature, Samsung Health, and Oura Ring do that among the major players. A few others like Natural Cycles use temperature, but they are all focused on fertility & conception.

That said, using an app like Drip that allows you to export data freely in a universal format can be incredibly valuable for personal analysis. You can find patterns in your data to make your own "predictor" or determine whether certain medications or lifestyle changes were effective. It can also be helpful at your next doctor visit.

[1] https://www.linkedin.com/posts/thefemalelead_wendi-aarons-a-...