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Founding batches, November and December 2026: half price on the PCOS Reset Cohort. See the cohort →
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Add the start dates of your last two to six periods to see your average cycle length, how much it varies, and roughly when your next period may start.
Written by the Femwise editorial team · Updated 7 October 2026 · Our reviewers check the medical facts on this page; they don't review or endorse our products.
Quick answer
In adults, a menstrual cycle usually lasts 21 to 35 days. Cycles that are often shorter or longer, that change by more than about a week each month, or periods that stop for three months are worth discussing with a doctor. They can be a sign of PCOS (now called PMOS), a thyroid problem or other conditions.
01
Day 1 is the first day of real bleeding, not spotting.
02
Count up to the day before your next period starts. That number is your cycle length.
03
Track at least three cycles before you trust the average.
04
Stress, illness and travel can shift a cycle; one odd month is common.
⚠ When to see a doctor
See a doctor if your period hasn't come for three months; if your cycles are often shorter than 21 or longer than 35 days; if you bleed for more than seven days or soak a pad every hour or two; if you bleed between periods or after sex; or if period pain stops you doing everyday things.
Questions
Is a 40-day cycle normal?
An occasional long cycle can happen with stress, illness or travel. If your cycles are often longer than 35 days, see a doctor: it can be a sign of PCOS or a thyroid problem, among other causes.
Can stress delay my period?
Yes. Stress, illness, travel, big changes in weight or exercise, and some medicines can all shift a cycle. If it keeps happening, see a doctor.
Why doesn't it show my fertile days?
Predicting fertile days from dates alone is unreliable, especially when cycles are irregular, so it isn't a safe way to plan or avoid a pregnancy. Talk to a doctor about family planning.
Do you save my dates?
No. Your dates stay on your phone, and only if you tick "Remember on this phone".
