First period
The first two years look nothing like the adult pattern
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Immaturity of the hypothalamic–pituitary–ovarian axis means many early cycles are anovulatory. Without ovulation there is no corpus luteum, no progesterone, and nothing to withdraw on a schedule — so the lining builds under estrogen alone and sheds when that estrogen dips or when the lining outgrows its support. The result is bleeding at unpredictable intervals, and it is normal maturation rather than disease. Applying the adult FIGO range of 24 to 38 days to a 13-year-old will label ordinary physiology as a disorder.
| Parameter | Adolescent | Adult (FIGO System 1) |
|---|---|---|
| Cycle frequency | 90% of cycles fall between 21 and 45 days. Mean interval about 32 days in the first year. | 24 to 38 days |
| By the third year | 60 to 80% of cycles are 21 to 34 days — the adult pattern is established. | — |
| Duration | 2 to 7 days at the first menses | 8 days or fewer |
| Longest acceptable gap | Remaining without a period beyond 90 days is statistically uncommon — 90 days is the 95th percentile for cycle length — and warrants evaluation even once. | Absence of bleeding for 6 months = amenorrhea |
Adolescent parameters: ACOG Committee Opinion No. 651, Obstet Gynecol. 2015;126(6):e143–e146, endorsed by the American Academy of Pediatrics. Adult parameters: Munro MG, Critchley HOD, Fraser IS. Int J Gynaecol Obstet. 2018;143(3):393–408.
The same committee opinion makes the broader argument this site is built around: the menstrual cycle should be treated as a vital sign, asked about at every visit once periods have started, because abnormal patterns in adolescence can be the first indication of polycystic ovary syndrome, thyroid disease, an eating disorder, a bleeding disorder, or other systemic illness.