Irregular Periods: When Normal Variation Is Worth a Conversation

Most of the periods people call irregular are ordinary variation. Across 612,613 cycles, a woman's own cycle length moved by a mean of 2.6 days from one cycle to the next, so a period that arrives three days early is the common case rather than a warning. The word has a narrower meaning than the way it is used: the NHS draws the line at a gap under 21 days or over 35, ACOG at a cycle length that varies by more than 7 to 9 days, and NICHD reports that 14% to 25% of women have irregular cycles. Which side of those lines your own pattern sits on is not something anyone can judge from memory, and that is the honest limit of this page.

The word does most of the damage. "Irregular" gets used for a period that came four days early, and it is also the clinical term for a pattern worth investigating. Nothing in the gap between those two uses ever gets explained. What follows is the measured baseline, the three definitions that mean something and who published each, the two stretches of life where irregularity is expected rather than surprising, and the points at which it is worth asking somebody.

The variation everybody has

Start with what a cycle does when nothing at all is wrong. In a study of 612,613 ovulatory cycles the mean length was 29.3 days, and only 13% were exactly the textbook 28. More useful than either of those: the mean variation in a single woman's own cycle length was 2.6 days from one cycle to the next.

That is the argument, so it is worth reading twice. The average cycle is not a fixed number that occasionally slips. It moves by a couple of days as a matter of course, in women whose cycles nobody would call irregular. A period two or three days off the date you expected is the cycle doing its ordinary thing.

The NHS puts the same fact in words rather than numbers: for most women a period comes around every 28 days, and it is common to be more or less frequent than that, ranging from every 21 days to every 35. What that band covers, and why one number was never going to describe it, is the whole of what counts as a normal cycle length.

Where the line is actually drawn

Three publishers, three definitions. They are not describing quite the same thing, which is worth knowing before you measure yourself against the one you half-remember.

ACOG's line is the one that catches people out, because it is about movement rather than length. A 26-day cycle followed by a 34-day one sits inside the NHS's band at both ends, and the two are 8 days apart from each other. By the NHS's definition that is not irregular. By ACOG's it is the sort of thing to mention. Neither is wrong. They are answering different questions, and only one of them can be answered by looking at a single cycle.

The six patterns NICHD names

NICHD's list is the most concrete of the three, and it is the useful one to keep because every item can be checked against a written record rather than against a memory. It counts as menstrual irregularity:

• no period by age 16, or periods stopping for at least 3 months when you are not pregnant;
• infrequent periods, more than 35 days apart;
• frequent periods, less than 21 days apart;
• bleeding that exceeds 8 days;
• cycle-to-cycle variation of more than 20 days;
• bleeding between periods.

Look at the fifth one against the first number in this article. Twenty days of cycle-to-cycle variation, against the 2.6 days of movement the average woman has. That gap is the distance between a cycle behaving normally and the pattern this list was written to catch, and it is much wider than the way people talk about their periods suggests.

Two stretches where irregular is expected

The NHS names both of them, and naming them is more use than any reassurance. Irregular periods are most likely when periods first start and when nearing the menopause. In those two stretches an unpredictable pattern is the expected thing rather than a departure from it.

The first has been measured closely. ACOG reports that 90% of adolescent cycles fall between 21 and 45 days — a much wider band than the adult one — and that by the third year after a first period, 60 to 80% of cycles run 21 to 34 days, as is typical of adults. So the settling is real, it takes years rather than months, and a fourteen-year-old comparing her cycles with her mother's is comparing two different things.

None of that makes the two stretches exempt from the thresholds below. It means an irregular pattern in either of them is common, not that it cannot be worth asking about.

When it is worth asking somebody

These are the lines the publishers themselves draw. They are more useful than "if you are worried", because you can check them against your own dates tonight.

• The NHS says to speak to a GP if your periods are irregular, or if your normal pattern changes. The second half of that matters as much as the first. A pattern that has always been all over the place and a pattern that changed this year are different pieces of information, and only you have the second one.
• The NHS says to see a GP if you have missed three periods in a row, or if your periods have not started by the time you are 15.
• ACOG says to seek emergency medical care right away if you are changing pads or tampons every hour for more than 2 hours in a row and also have chest pain, shortness of breath or lightheadedness.

What sits behind an irregular pattern is a question for a doctor, a nurse or a pharmacist rather than for a web page, because it needs your history and often a test. One of the conditions a clinician considers is worth naming, since it is the name most readers arrive with: the NHS now calls it polyendocrine metabolic ovarian syndrome, PMOS, which used to be called polycystic ovary syndrome, or PCOS. The NHS describes it as a hormone condition whose main symptoms include irregular periods or long gaps between periods, with symptoms that usually start at puberty and continue until the menopause.

Naming a condition is not the same as having one. Irregular periods have many ordinary explanations as well as medical ones, and nothing on a screen can tell you which applies to you — that is the difference between a page and an appointment. The ordinary explanations for a single late month are set out in why a period is late, and the one everybody reaches for first is examined, with what the research does and does not support, in what stress can actually do to a cycle.

You cannot judge a pattern you have not measured

Here is the practical end of all this. Almost every conversation about irregular periods runs on memory, and memory is terrible at this. It keeps the two months that felt wrong and quietly drops the eight that were unremarkable. Ask somebody how long her cycle is and you will usually get a single number, delivered with a shrug; ask her to prove it and there is nothing to look at.

Three or four recorded start dates fix that, and it is a low enough bar to clear this week. Four start dates give three cycles, which is enough for an average, a shortest and a longest — and the difference between the last two is the number every definition above is actually asking about. The cycle length calculator works those out from the dates and prints how many cycles they came from, which is the part that tells you how much weight the average can carry. If you would rather keep it on paper and off any screen, the printable period tracker is a year on one sheet.

Two things follow from having those numbers. The first is that "am I late" becomes answerable, because late is a distance from your own range rather than from somebody else's 28 days — which is what the period late calculator measures. The second is that if you do end up in a GP's room, you arrive with an average, a range and a count of cycles instead of "it has been all over the place lately". Ten minutes of an appointment can be spent on what that record shows rather than on trying to reconstruct it.

None of this is a diagnosis, and none of it is a reason to worry about a cycle that moves. It is the one thing this genre almost never says: the pattern is the evidence, and a pattern nobody wrote down does not exist yet.

If the irregularity is the ordinary kind that comes with periods being new, tracking your first year of periods covers what to write down in those months and which two thresholds are worth carrying. And the point at which any of this is worth an appointment rather than a note is set out, tier by tier, in when to see a doctor about your period.

And if the pattern changed after coming off a hormonal method, that is a different question with its own evidence — periods after stopping the pill sets out what was actually measured about how long cycles take to look like themselves again.