How accurate are period and cycle predictions?
This is not a contraceptive method
Calendar-based prediction is not contraception, and treating it as one is a well-documented route to an unintended pregnancy. Fertility awareness methods that do work require daily physiological measurement and proper instruction — they are not what a calendar app does.
For contraception, talk to a healthcare professional about methods that have actual effectiveness data behind them.
Where the data goes, which is worth asking
Cycle data is among the most sensitive information on a phone. Data held on your device is exposed only if the device is. Data synced to a company's servers is subject to that company's policy, its security, and whatever legal requests it receives.
Read what an app says it collects, and check whether it uses third-party analytics or advertising SDKs — those are separate recipients from the app maker itself. On-device storage removes the question entirely.
What is worth recording beyond dates
Symptoms and their timing. Patterns across several months are hard to notice and easy to record, and that history turns "it has been a few months" into a specific account in a medical appointment.
Changes worth raising with a clinician include cycles becoming markedly irregular, unusually heavy bleeding, or severe pain. An app can show you the pattern; interpreting it is a medical question.
Our iOS app for exactly this. Runs on your device — no account, no tracking.