Every period tracker I tried wanted to predict my next cycle with "AI." Download a 200MB app, create an account, let the model learn my body. Meanwhile my actual question was simple: based on my last few cycles, roughly when should I expect the next one?
That is not a machine learning problem. It's a statistics problem.
What the "AI" actually does
Cycle prediction, at its core, is: take your last N cycle lengths, compute a central estimate, add a window. A mean or median over your recent history, maybe weighted toward recent cycles. That's it. You don't need a neural net to average five numbers.
The apps that market "AI-powered predictions" are mostly doing this same arithmetic, then wrapping it in language that sounds smart. The model isn't the point — the point is that you're creating an account and uploading your cycle history to their servers, where it becomes training data and a retention hook.
What I built instead
I made Mori, a free Android period tracker that keeps everything on the phone. No account, no email, no cloud by default. The prediction is plain statistics on your own logged dates:
- Take up to your 6 most recent cycles
- Median cycle length as the central estimate
- A date range, not a single fake-precise day
- A confidence label that drops when your history is short or irregular
When there's not enough data, it says "not enough data" instead of inventing a confident guess. Because it's deterministic, I can actually test it: feed the same cycle history, get the same answer, every time. No model drift, no silent retraining, no "the algorithm changed."
The uncomfortable part
Predictions from a phone-side heuristic are less magical than what a billion-parameter model might promise. But cycle prediction was never the hard part. The hard part is that the data is sensitive, the app should work in airplane mode, and you shouldn't have to trade an account for basic arithmetic.
If your tracker needs an ML model to tell you when your period is probably coming, it's solving a different problem than yours.
Mori is free, one-time-purchase optional sync, APK direct download. Source of truth is on your device, not on our servers.
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