First period
The age is falling, and the guidance has not caught up
2 min read
Two current sources, two different pictures
Stable, or declining?
ACOG, 2015. Median age at menarche has remained relatively stable, between 12 and 13 years, across well-nourished populations in developed countries.
Apple Women's Health Study, 2024. Among 71,341 US participants, mean age at menarche fell from 12.5 years in those born 1950–1969 to 11.9 years in those born 2000–2005. Menarche before age 11 rose from 8.6% to 15.5%; before age 9, from 0.6% to 1.4%. Time to regularity lengthened — the proportion reaching regular cycles within two years fell from 76% to 56%. In a subset providing body mass index at menarche, roughly 46% of the decline was attributable to higher BMI. Trends were present across all sociodemographic groups but most pronounced among participants identifying as Black, Hispanic, Asian or mixed race, and those reporting low socioeconomic status.
Both are current. The ACOG statement dates from 2015 and describes stability over the preceding decades; the 2024 analysis is larger, more recent, and relies on self-recall in an app-based cohort that is not population-representative. Neither is wrong on its own terms. This site reports the disagreement rather than resolving it, because the practical consequence is real: if the average is drifting earlier and cycles are taking longer to settle, then guidance written around a stable median will increasingly under-recognise both early menarche and prolonged irregularity.
ACOG Committee Opinion No. 651, Obstet Gynecol. 2015;126(6):e143–e146. Wang Z, Asokan G, Onnela JP, et al. JAMA Netw Open. 2024;7(5):e2412854. Full citations on the references page.
Practical implication. Self-recalled age at menarche in an app-based volunteer cohort carries recall and selection bias, and the absolute shift is modest. The more actionable finding is the lengthening time to regularity, which is prospectively relevant: a larger proportion of adolescents will remain irregular beyond two years, and distinguishing extended physiological maturation from incipient PCOS or hypothalamic dysfunction in that group is a genuine clinical problem that existing thresholds handle poorly.