How often do periods come? Why the date moves each month
The short answer
Most adults get a period every 21 to 35 days, counted from the first day of one period to the first day of the next. The average is about 28 days, which is why people say “once a month”. But a cycle is not tied to the calendar month, so the date your period starts moves from month to month.
When in the month do periods start?
There is no fixed date. Your period starts when your cycle ends, and cycles are counted in days, not months:
- With a 28-day cycle, your period comes about 2 to 3 days earlier each calendar month (months are 30 or 31 days long).
- With a 30 or 31-day cycle, it lands on roughly the same date each month.
- With a 24-day cycle, it moves about a week earlier each month, and some months you’ll have two periods.
Two periods in one calendar month is normal if your cycle is short. It is the gap between periods that matters, not the month.
How often should you get your period?
| Stage | Normal gap between periods |
|---|---|
| First 1 to 2 years after the first period | 21 to 45 days (ACOG) |
| Adults | 21 to 35 days (NHS) |
| Approaching menopause (usually 40s) | Gaps get longer and more variable |
The NHS describes periods as irregular when the gap is regularly shorter than 21 days or longer than 35 days, or when the length changes a lot from cycle to cycle. In teens, that range is wider. Our guides cover a 21-day cycle and a 40-day cycle in more detail.
When will my period start?
The best estimate is: the first day of your last period, plus your average cycle length. If your last three cycles were 27, 29 and 28 days, use 28. Our next period calculator does this for you, and the Nepali (BS) calculator gives the date in Bikram Sambat as well.
Signs it is close (in adults with settled cycles): sore breasts, bloating, cramps, mood changes, or a day or two of light spotting. These vary a lot between people, so they’re clues, not a schedule.
If you have never had a period, see signs your first period is coming.
My period came 15 days early
A single early period is usually nothing: stress, illness, travel or a change in sleep can shift ovulation and bring the period forward. Bleeding about two weeks after a period can also be ovulation spotting (light, a day or two) or breakthrough bleeding if you’re on hormonal contraception.
It is worth a doctor’s visit if:
- gaps are regularly shorter than 21 days
- you bleed between periods more than once or twice
- bleeding is heavy or lasts more than 7 days
- you could be pregnant and have bleeding with pain
Why cycles change over time
- Teens: ovulation doesn’t happen every cycle yet, so gaps vary. Cycles usually settle within about two years.
- Stress, weight change, heavy exercise, illness: can delay ovulation and make the cycle longer that month.
- Hormonal contraception: sets its own pattern (the pill, implant or hormonal IUD).
- After pregnancy or while breastfeeding: periods may stop for months and return irregularly.
- Conditions like PCOS (now called PMOS) or thyroid problems: cause long or irregular gaps. See why is my period late.
- 40s onwards: cycles become irregular before menopause.
Keep track to see your own pattern
Logging day 1 of each period for three or four months tells you your own cycle length and how much it varies. That is more useful than any general rule, and it is exactly what a doctor will ask about. Cycella logs a period in a few taps with no account, and keeps the data on your phone; it is free on Google Play for Android.
Sources
Try the Next period calculator, or see Cycella in Nepal. Cycella is a free, private Android period tracker, now on Google Play.
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