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How long is a normal menstrual cycle?

By Chris Ayers 7 min read

The number everyone knows is 28. It is on the pill packet and in the textbook diagram. It is also only an average, and a fairly loose one. If you want to know what a normal menstrual cycle length actually is, the better answer is a range, plus a second range for how much it is allowed to move from one month to the next. Both come from the clinical guidance, and two very different datasets fill in the detail, one small and careful, one enormous.

The clinical range

The American College of Obstetricians and Gynecologists puts it this way in its patient guidance on abnormal bleeding: the normal length of the menstrual cycle is typically between 21 and 35 days, and a normal period generally lasts up to 7 days.

The NHS says the same thing in slightly different words. Periods come around every 28 days for most women, ranging from every 21 days to every 35 days. The period itself can last between 2 and 7 days, usually about 5. For what it is worth, the NHS also gives the amount of blood lost as about 20 to 90 milliliters, roughly 1 to 5 tablespoons.

So the adult range is 21 to 35 days, counted from the first day of one period to the day before the next. A 24-day cycle and a 33-day cycle are both inside it. Neither is more normal than the other.

What a careful small study found

In 2006, Fehring and colleagues followed 141 healthy women through 1,060 cycles for a paper in the Journal of Obstetric, Gynecologic and Neonatal Nursing. The mean cycle was 28.9 days, with a standard deviation of 3.4 days, and 95 percent of cycles fell between 22 and 36 days. That is almost exactly the clinical range, found independently in real cycles rather than written down as a rule.

The more interesting number in that study is about change within a single woman. Cycle length varied by more than 7 days in 42.5 percent of the participants. Put plainly: close to half of healthy women had at least one pair of cycles that differed by more than a week. The authors described “considerable normal variability” and traced most of it to the first half of the cycle, the follicular phase, which I explain in the four phases article.

This is the fact I wish was printed next to the 28. A cycle that is 26 days one month and 34 the next is not a broken cycle. It is a common one.

What a huge dataset found

In 2023, Li and colleagues published cycle data from the Apple Women’s Health Study in npj Digital Medicine: 165,668 cycles from 12,608 participants in the United States. With that many cycles they could ask a question the small study could not: how does cycle length change with age?

The answer has a clear shape. Mean cycle length got shorter with age until about 50, then longer. And variability, the amount cycles bounce around from month to month, followed a U-shape. It was lowest at ages 35 to 39. Compared with that low point, it was about 46 percent higher under age 20, about 45 percent higher at ages 45 to 49, and about 200 percent higher over 50.

So “normal” depends a little on how old you are. A teenager’s cycles are expected to wander more than a 37-year-old’s, and so are the cycles of a woman in her late forties. The guidance for adolescents, from ACOG’s committee opinion on menstruation in girls, describes a wider typical range of 21 to 45 days, and notes that by the third year after a first period, 60 to 80 percent of cycles are 21 to 34 days long, “as is typical of adults.” Cycles settle. They do not start out settled.

What this means for the number in your app

I build a cycle app, so here is the practical side. If your app asks for your cycle length and you type 28 because that is the number you know, every prediction starts from an assumption that may be a week off for you. Ferrabelle starts from population research conditioned on your age, which is a better guess than 28 but still only a guess, and then replaces it with your own history as you log cycles. The age-adjusted start is there precisely because of findings like Li’s: a 19-year-old and a 47-year-old should not get the same default.

The variability finding matters just as much. A prediction to the exact day promises more than this much normal variation allows. Ferrabelle shows a window and a confidence label instead, and on the accuracy page I publish how often it lands within 1 and 2 days on a public dataset, along with where it does worse.

When to talk to a doctor

The ranges above are also the lines that clinicians use to decide what is worth a closer look. ACOG’s list of bleeding patterns that count as abnormal includes cycles longer than 35 days or shorter than 21 days, “irregular” periods where the cycle length varies by more than 7 to 9 days, bleeding that lasts more than 7 days, bleeding or spotting between periods or after sex, not having a period for 3 to 6 months, and any bleeding after menopause.

Note the second item. The 7-day swing that Fehring found in 42.5 percent of healthy women sits right at the edge of ACOG’s threshold. That is not a contradiction. It means an occasional swing of a week is common, and a pattern of swings beyond 7 to 9 days is what the guidance says to raise with a doctor. The difference between the two is exactly what a few months of logging shows you.

ACOG also lists an emergency sign: if you are changing pads or tampons every hour for more than 2 hours in a row, and you also have chest pain, shortness of breath, and feel lightheaded or dizzy, seek emergency medical care right away.

I have written up the irregular-cycle guidance in more detail, including the NHS list of reasons to see a GP, in irregular periods: what counts, and when to see a doctor. The short version is the same as this article’s: if your cycles fall outside 21 to 35 days, or change in a way that is new for you, talk to your doctor. I am a software developer, and the most useful thing my app can do in that situation is hand you a clean record of what happened and when.

This is general information, not medical advice. Talk to your doctor about your own health.

Ferrabelle is in a closed test on Google Play right now. If you want a cycle app that starts from your age instead of from 28, you can join at ferrabelle.com/testers.

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