MENSTRUAL-CYCLE PATTERNS AND RISK OF BREAST-CANCER

Menstrual cycle characteristics may reflect underlying endocrine patte rns that influence the risk of breast cancer. Most previous studies of menstrual function and breast cancer risk have used retrospective rep orts of menstrual bleeding, which may be unreliable. To examine this a ssociation, the authors conducted a mail survey among 997 women who ha d recorded menstrual events prospectively over as many as 50 years, be ginning in 1934. Compared with women with a median menstrual cycle len gth of 26-29 days, women who had cycles of extreme length at ages 25-2 9 years had a nearly twofold increased incidence of breast cancer (for a median cycle length of less than 26 days, adjusted relative risk (R R) = 1.9, 95% CI 0.9-4.1; for greater than or equal to 34 days, RR = 1 .9, 95% CI 0.9-3.9). Statistical adjustment was made for age, family h istory of breast cancer, parity, age at menopause, age at first pregna ncy, and Quetelet index (weight (kg)/height (m)(2)). Adjusting for age and other potential confounders and restricting the analysis to women who did not use hormones, women who experienced either a lesser (<150 ) or a greater (>350) cumulative number of cycles had an increased inc idence of breast cancer (adjusted RR = 1.9, 95% CI 0.3-10.6, and RR = 1.8, 95% CI 0.5-6.0, respectively) compared with women who experienced 150-350 cycles. The findings are discussed in the context of current hormonal theories of breast cancer etiology.