Plum predicts from the periods you log and the cycle length you give it at setup. It doesn’t read your temperature, your heart rate or a test strip. Every period you add makes the next prediction a little better.
Your next period
Before you’ve logged three cycles, Plum starts from the cycle length you gave it during setup. If you skipped that, it uses a typical length for your age, which runs from about 28 to 31 days across age groups. Each cycle you log then nudges the estimate towards your own cycle length.
From three logged cycles on, the prediction is the median of your recent cycles, up to the last six. One odd cycle barely moves a median. If a cycle runs more than a week past your usual length, Plum treats it as a possible late log and leaves it out of the median. That way one forgotten period doesn’t stretch every forecast after it.
When your cycles vary by more than about five days, Plum switches to the median of the last three, so the date follows where your cycle is now. The calendar still shows one day. Insights shows how much your cycles vary, as plus or minus a number of days, so you can see how far to trust it. The irregular periods guide covers what makes cycles swing and when it’s worth a doctor’s visit.
How long your period lasts
The predicted length is the median of your last three periods, or the length you gave during setup before there are three. When you log the first day of a period, Plum fills in the rest of the usual days for you, and you can tap any of them to correct it. A gap of up to two unlogged days inside a period still counts as the same period.
Ovulation and the fertile window
Plum counts back from the next expected period. It takes the stretch from ovulation to the next period, the luteal phase, as 14 days for cycles of 25 to 32 days. It uses a little less for shorter cycles and a little more for longer ones. The day that lands on is the estimated ovulation day. The fertile window is the five days before it plus the day itself, six days in all, because sperm can live about that long.
That’s an estimate from dates, and it carries the same uncertainty as the period prediction it hangs off. Our fertile window guide explains what the estimate can and can’t tell you, and why it isn’t contraception. For how this compares with other apps, and what studies found about calendar-based predictions, read how accurate period tracker apps are.
Birth control
If you set up a hormonal method in Me, then Birth Control, Plum lowers its confidence and softens its wording. Because those methods change or stop ovulation, it also turns the fertile window and ovulation display off by default. Period logging and period predictions keep working the whole time.
