How Moonkeep predicts your period

In short: Moonkeep learns from your own cycles, gives more weight to recent ones, uses your temperatures and ovulation tests when you log them, and shows an honest window with a confidence level. Tap Why this date? in the app to see each step for your own data.

1. Your own cycle history

A cycle runs from the first day of one period to the day before the next. Moonkeep looks at the cycles you've logged, and gives recent cycles more weight than old ones, so changes show up quickly. Until you've logged three cycles, it blends in the usual length you gave when you started.

2. A window, not a promise

Very few people have exactly the same cycle every month. Moonkeep measures how much your cycles vary and shows your next period as a likely date with a window around it. The window is narrow when your cycles are steady and wider when they aren't. With Irregular cycles turned on, for example with PCOS, the window widens again instead of pretending to know.

3. Confidence that grows

Each prediction comes with a confidence level: low, medium or high. It rises as you log more cycles and as they become more regular. Moonkeep also checks itself: it re-predicts your past cycles from the data before each one and tells you how close it got.

4. Ovulation and your luteal phase

The time from ovulation to your next period, the luteal phase, is usually steadier than the rest of the cycle. When you log basal body temperature, Moonkeep looks for a sustained rise: three readings above the previous six. A positive ovulation test is used too. Over several cycles Moonkeep learns your luteal length instead of assuming 14 days, which sharpens the next-period date.

5. The four phases

• Menstrual: your period. Hormones are at their lowest.
• Follicular: from the end of your period to just before ovulation. Oestrogen rises.
• Ovulatory: a few days around ovulation.
• Luteal: from after ovulation until your next period. Progesterone rises, then both hormones fall.

Moonkeep fits these phases to your own cycle length, and the phase guide suggests gentle movement and food ideas for each one. They're general wellness ideas, not medical advice.

6. Body patterns (Plus)

When you log symptoms, Moonkeep looks at your last six cycles for symptoms that keep showing up at the same point, such as headaches a day or two before your period. It only reports a pattern once it has seen it in at least three cycles and in at least half of the cycles it checked.

What Moonkeep is not

Moonkeep is a wellness and record-keeping app. It is not a medical device and not a method of contraception. Fertile days are an estimate. Never use them to prevent pregnancy. If anything about your cycle worries you, talk to a doctor, midwife or pharmacist.

Apple Health

With Apple Health turned on, Moonkeep reads your menstrual flow, basal body temperature and ovulation test history and uses it exactly as if you had logged it in the app. What you log in Moonkeep is saved back to Health. If the same day is in both, what you logged in Moonkeep wins.