A period tracker is a calendar with some arithmetic. Knowing the arithmetic helps you judge how much to trust it.
What it does
- You log the first day of each period, and often flow, symptoms and mood.
- The app calculates your average cycle length from past entries.
- It adds that length to your last start date to predict the next period, and estimates a fertile window from the same average.
Why predictions can be wrong
- Few entries. With one or two cycles the average is unreliable.
- Irregular cycles. Stress, illness, travel, weight change, breastfeeding, hormonal medication and conditions such as PCOS can shift timing.
- Missed logs. A forgotten entry skews the average.
What it cannot do
- It cannot prevent or confirm a pregnancy. Use a pregnancy test and speak to a clinician.
- It cannot diagnose a medical condition.
- It cannot replace medical advice. When something feels wrong, see a doctor.
Getting more useful results
Log consistently for at least three cycles, note symptoms that matter to you, and export or show your history at appointments. Before you start, read the privacy checklist for cycle tracking apps.
This is general information, not medical advice.