Age at menarche and breast cancer risk in nulliparous women.

Early age at menarche is a known risk factor for breast cancer, some inconsistency in the literature not withstanding. Relative risks for an early menarcheal onset as compared to a late onset vary from 1.0 to 1.9. To avoid (residual) confounding by parity-related factors a case-control study was conducted among nulliparous women. 135 cases and 540 controls were selected from two population-based screening projects for breast cancer in The Netherlands. Four controls were matched to each case for year of birth, for screening-centre, and for number of screening examinations. Women with a menarcheal age of 10 or 11 years showed a 2.2 times (95% confidence interval: 1.2-4.0) higher risk for breast cancer compared to women who had their first menstrual period at 12 years of age or older.