Age at menarche is defined as age at the first menstrual period (in years). This information was retrospectively ascertained by recall in the baseline questionnaires. In the NHS, the question was open-ended: “At what age did your menstrual periods begin?___ years of age”. In the WHS, the question asked, “At what age did your menstrual periods begin?” Response categories were: “9 or younger; 10; 11; 12; 13; 14; 15; 16; 17 or older”. We excluded women whose age at menarche was reported as 18 years or greater in the NHS from the analysis (N=2), as these women were more likely to have a pathological cause outside the spectrum of normal variation.
