The basics
What counts as normal
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The international standard is FIGO System 1, which describes menstrual bleeding along four separate axes rather than as one global impression. These are the numbers clinicians use.
| Axis | Normal | Outside normal |
|---|---|---|
| Frequency | Every 24–38 days | Under 24 days = frequent · Over 38 days = infrequent |
| Duration | 8 days or fewer | Over 8 days = prolonged |
| Regularity shortest to longest cycle over a year | Ages 26–41: varies by 7 days or less Ages 18–25 or 42–45: varies by 9 days or less | Greater variation = irregular |
| Volume | Not excessive by the patient's own judgment | Heavy, or light, as reported |
Source: Munro MG, Critchley HOD, Fraser IS; FIGO Menstrual Disorders Committee. Int J Gynaecol Obstet. 2018;143(3):393–408. Confirmed in Jain V, Munro MG, Critchley HOD. Int J Gynecol Obstet. 2023;162(Suppl 2):29–42.
Note what is not in that table. Nothing requires a cycle to be 28 days. In a digital cohort of more than 600,000 cycles the average was about 29.3 days, with wide spread around it. A 33-day cycle that arrives reliably is normal. A 28-day cycle that swings to 40 and back is not.
The cycle is also treated as a vital sign — a readout of endocrine and general health alongside pulse and blood pressure. Cycles that go missing, or that change character and stay changed, are information about the body as a whole, not only about the uterus.
Age changes the answer. Cycles are most variable in the first years after menarche and again in the years before menopause. In between, they are at their most consistent. Any assessment that ignores age will mislabel a normal adolescent and a normal 45-year-old as abnormal.