AGE AT MENARCHE AND BREAST-CANCER RISK IN NULLIPAROUS WOMEN

Early age at menarche is a known risk factor for breast cancer, some i nconsistency 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-c ontrol study was conducted among nulliparous women. 135 cases and 540 controls were selected from two population-based screening projects fo r breast cancer in The Netherlands. Four controls were matched to each case for year of birth, for screening-centre, and for number of scree ning examinations. Women with a menarcheal age of 10 or 11 years showe d a 2.2 times (95% confidence interval: 1.2-4.0) higher risk for breas t cancer compared to women who had their first menstrual period at 12 years of age or older.