How estimates work

Hemogoblin 1.1

Every estimate comes from what you log, calculated on your iPhone with the rules below. No black box, no server, no guessing from other people's data.

Periods

• A period starts on a day with light, medium or heavy flow after two or more days without one. Spotting doesn't start a period, and days you didn't log count as no flow.
• Your usual period length is the median of the periods you've logged. One-day periods are skipped, since they usually mean only the first day was logged. Until there's history, it's the length you gave at setup, or 5 days.

The next period

• The date: your last period's start plus the median of your last six cycle lengths (with an even number, the upper of the two middle values). With only one period logged, it uses the cycle length you gave at setup, or 28 days.
• The likely range: from your shortest to your longest of those recent cycles.
• Odd cycles: once there are three cycles to compare, a cycle over 1.5 times your median (probably a missed log) or under 0.6 times (probably bleeding mid-cycle) is flagged and left out of the range, so one forgotten period doesn't throw off the next six months.
• Irregular cycles: when recent cycles differ by more than 7 days, the app tells you the date is rough.

Phases and the fertile window

• Estimated ovulation: cycle day (cycle length minus 14), so day 14 of a 28-day cycle, and never sooner than 3 days after your period. Fourteen days is a textbook average for the time from ovulation to the next period; yours may be different. With body signals, a temperature shift moves it (see below).
• Ovulation phase: the estimated day, plus a day either side.
• Fertile window: 5 days before the estimated ovulation through the day after it, never during your period.
• Follicular phase: after your period until the ovulation phase. Luteal phase: after the ovulation phase until your next period. Late: past the expected start.

Temperature shifts

Only if you turn on body signals and Apple Health has nightly temperatures.

• One temperature a night. A thermometer's readings are used for any cycle with six or more of them; otherwise Apple Watch's sleeping wrist temperature. The two are never mixed in one cycle, because they read differently.
• A shift is three readings within four nights, each at least 0.2 °C (0.36 °F) above the highest of the six nights before them, with at least five of those six recorded. A night more than 1 °C above them is skipped as a fever, a drink or a hot room. If two of the next three readings drop back, the shift is withdrawn.
• What it means: a sustained rise usually follows ovulation by a day or two, but it isn't proof of ovulation. The app puts the estimated ovulation on the night before the shift, give or take two days.
• Your own luteal length. Once three finished cycles each have a shift, the median number of days from estimated ovulation to the next period becomes yours. When the current cycle shows a shift, the next period is estimated from it plus that length, with a range from your shortest to your longest (at least two days either side).
• Resting heart rate before and after the shift is shown as a comparison only. It doesn't change any date.

Patterns and trends

• A symptom or mood becomes a pattern once it's been logged at least 3 times, across at least 2 cycles, and at least 60% of those logs fall within 2 days of the same point: a cycle day, or a number of days before a period.
• Trends compare your last three finished cycles with the three before them.
• Findings about temperature and heart rate appear only once three cycles agree.

How close it's been

Insights replays the rule above on your own history: for each past period, what the app would have estimated using only what was logged before it, and how far off that was. A cycle that looks like a missed log isn't counted against it.

Worth mentioning to a doctor

Once there are three periods to go on, Insights looks back over the last six months for a few patterns in your own logs that are worth bringing up with a doctor. It says what it found in your numbers. It never names a condition, and it isn't a diagnosis. The limits follow FIGO's definitions for menstrual bleeding and Apple Health's cycle deviations.

• Long periods: two or more periods with 10 or more days of bleeding (counted as the Periods card counts them), once each has ended.
• Spotting between periods: spotting in two or more cycles, at least three days after a period and more than five days before the next.
• A long gap: 90 days or more without a period, as long as you logged something at least every 30 days through it. Otherwise it's as likely a missed log or a break from the app.
• Long or short cycles: most recent cycles over 38 days (logged through, as above), or most of three or more under 24 days, leaving out cycles that look like a mid-cycle bleed.
• Pain: 8 or more out of 10 during three of your last six periods.

It stays quiet while estimates are paused, looks only at what came after estimates were last turned back on, waits two weeks after you start using the app, and says nothing about bleeding when your notes mention birth control, a pregnancy, breastfeeding or menopause. "Got it" sets a pattern aside until something newer adds to it, and you can switch it off in Settings. Nothing about it leaves your phone unless you share it.

What doesn't count

The moon phases on the calendar are there for fun. They don't affect any estimate. Neither do the curiosities.