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Irregular periods: what counts, and when to see a doctor

By Chris Ayers 6 min read

Most of the time, “irregular” is a feeling. Your period came early, or late, or it turned up in the middle of a training block when you were sure it was a week away. Doctors have a stricter meaning for the word, and it helps to know it before you worry, and before you shrug something off.

I am not a doctor. I write software. Everything below is quoted or closely paraphrased from two sources I trust for this: the American College of Obstetricians and Gynecologists (ACOG) and the UK’s National Health Service (NHS). The links are at the end, and if anything here differs from what your own doctor tells you, your doctor wins.

What doctors call normal

ACOG puts it plainly: “The normal length of the menstrual cycle is typically between 21 and 35 days. A normal menstrual period generally lasts up to 7 days.” The NHS uses the same window and calls periods irregular “if the gap between them is less than 21 days or more than 35 days,” with the average gap around 28 days.

Teenagers get a wider range. ACOG’s Committee Opinion 651 says cycles in adolescents are typically 21 to 45 days, and that by the third year after the first period, 60 to 80% of cycles are 21 to 34 days long, “as is typical of adults.” That same document is the one that calls the menstrual cycle a vital sign, and says clinicians “should ask at every preventive care or comprehensive visit for the patient’s first day of her last menstrual period and the pattern of menses.”

So a cycle of 27 days one month and 31 the next sits inside both sets of lines. The word irregular, in the medical sense, is for something bigger than that.

What ACOG calls abnormal

ACOG’s patient page on abnormal uterine bleeding gives this list. I have kept its wording.

The 7 to 9 day line is the one I had never heard of before I built this. It is the difference between a cycle that wanders a little and one a clinician wants to hear about.

Common causes, in their words

The NHS page on irregular periods lists these common causes: puberty, the start of menopause (usually between 45 and 55), pregnancy, hormonal contraception, losing or gaining a lot of weight, stress and anxiety, and exercising too much. It adds that conditions like polyendocrine metabolic ovarian syndrome (PMOS, which used to be called polycystic ovary syndrome or PCOS) or thyroid problems can cause irregular periods. The NHS page on missed or late periods adds sudden weight loss, being overweight and breastfeeding.

Two items on those lists matter for anyone who lifts: “exercising too much” and “sudden weight loss.” If you are training hard and cutting weight at the same time, and your periods have gone irregular or stopped, the NHS lists below still apply: irregular periods and 3 missed periods in a row are both reasons to see a GP. It is not something to train through quietly.

I am not going to tell you which cause applies to you. Neither should an app. A cycle tracker can show you the pattern; only a clinician can tell you what the pattern means.

When to see a doctor

The NHS irregular periods page says to see a GP if:

The NHS missed or late periods page says to see a GP if:

And ACOG gives one emergency line, which I will quote in full: “If you are changing pads or tampons every hour for more than 2 hours in a row, and you also have chest pain, have shortness of breath, and are lightheaded or dizzy, seek emergency medical care right away.”

What the app does with irregular cycles

Ferrabelle predicts worse when cycles are long or irregular, and it says so. On the public dataset I test it against, cycles in the regular range predicted at 1.79 days mean error, while long cycles (over 35 days) came in at about 2.8 days. So when your cycles vary, the app shows wider confidence windows “rather than falsely precise dates.” There is also a cycle tracking mode called “Perimenopause and irregular cycles” that shows predictions as a wider range.

Where a log earns its keep is the appointment itself. ACOG’s two questions, the first day of your last period and the pattern of your periods, are exactly what a few months of dates answer. The printable doctor report (a Pro feature) lists your period dates and cycle lengths and marks any cycle outside 21 to 35 days, so you are not scrolling through a calendar in the waiting room.

None of that is a diagnosis. It is a record, and the record is for your doctor to read.

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 to try it, you can join at ferrabelle.com/testers.

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