Menstrual factors in relation to breast cancer risk.

We evaluated menstrual factors in relation to breast cancer risk in a large, population-based, case-control study. Case women were ascertained through state-wide registries covering Wisconsin, Western Massachusetts, Maine, and New Hampshire; control women were randomly selected from driver's license and Medicare lists in each state. Information regarding menstrual characteristics was obtained through a telephone interview. The study population comprised 6888 breast cancer cases and 9529 control women. Because exogenous hormones influence menstrual cycle patterns, we repeated our analyses in a subgroup of women who had never used oral contraceptives or hormone replacement therapy. Our results indicate decreased breast cancer risk with menarcheal age of 15 years or more, relative to menarche at age 13; the relation was stronger among premenopausal [odds ratio (OR), 0.72; 95% confidence interval (CI), 0.57-0.91] as opposed to postmenopausal women (OR, 0.90; 95% CI, 0.80-1.03). Risk was slightly reduced among premenopausal women whose menstrual cycles did not become regular until at least 5 years after onset of menses, relative to those whose cycles became regular within 1 year (OR, 0.80; 95% CI, 0.63-1.02). There was no clear relation between breast cancer risk and irregular menstrual cycles, episodes of amenorrhea, or menstrual cycle length. Early menopause, whether natural or surgical, was associated with decreased breast cancer risk; surgical menopause before age 40 conferred the strongest protective effect (OR, 0.57; 95% CI, 0.47-0.71). We found no evidence of increased risk with late natural menopause (OR, 0.92; 95% CI, 0.80-1.06). Results in the subgroup of women who never used exogenous hormones were similar to those for the entire group.