Reference summary — not medical advice or a personal risk estimate.
Early onset of menstruation (before age 12)
Quick answer
Not IARC-classified — this is not a chemical or physical agent IARC rates. Recognized as a cancer risk factor based on observational evidence. Biological / developmental. Applies to people whose first menstrual period occurred before age 12.
Summary
Starting menstruation before age 12 is associated with a modestly higher lifetime breast cancer risk, linked to longer cumulative exposure to ovarian hormones.
What's not established
This reflects a population-level statistical association, not an individual risk figure. It does not establish a specific magnitude of increased risk for any one person, and age at menarche is not something a person can control or change.
Evidence sources
Every claim on this page traces back to one of these. Click through to read the primary document yourself.
Primary sources
NCI
NCI — Reproductive History and Cancer Risk
Last reviewed: October 2026
Official reference list
A government list some readers use alongside the primary evidence. Meyron is not affiliated with it. A chemical being listed does not by itself describe a risk from any particular product.
- California Prop 65 list
State of California's list of chemicals known to cause cancer or reproductive harm.