Tracking
How do period apps predict your next period?
Period apps look at how long your recent cycles were and use that to estimate when the next one will start. The more periods you log, the closer the estimate gets. Because cycles move by a few days from month to month, a good app shows a range of dates, not a single day.
What does the app actually use?
The main ingredient is your own history: the dates your periods started. From those it works out your usual cycle length and adds that to the start of your last period. Seventh uses the median of your recent cycles, which stops one unusual cycle from pulling the estimate off.
Why is a range better than a single date?
A study of more than 600,000 cycles from app users found a person’s cycle length usually shifted by a few days from month to month. A single date pretends to a precision the body doesn’t have. A window, like Nov 4 to 10, matches how cycles behave.
Why are the first predictions less accurate?
With one or two logged periods there is little to go on. The estimate starts from general averages: about 28 days, and a usual range of 21 to 35 days (NHS). Each new period you log makes it more yours.
What about irregular cycles or PCOS?
The NHS counts a gap of less than 21 or more than 35 days as irregular. When cycles vary a lot, any estimate is less certain. In Seventh, you can say your cycles are irregular or that you have PCOS, and the window widens to match.
How accurate are fertile days and ovulation dates?
Less accurate than period dates. Ovulation moves around from cycle to cycle, and the app can only estimate it. Never use these estimates as contraception.
Frequently asked questions
Can an app tell me why my period is late?
No. An app can show that a period is later than your pattern, but only a test or a doctor can tell you why. See why a period is late.
Do I have to log every day?
No. The start date of each period matters most. Logging symptoms and mood adds detail for insights and for your doctor.
Is a prediction the same as a diagnosis?
No. Predictions are estimates from your past cycles. They are not a medical diagnosis.
Sources
Seventh is a free period and cycle tracker for iPhone and iPad, by doogud. This article is general information, not medical advice. If something feels wrong, see a doctor.
