The role of reproductive factors and use of oral contraceptives in the aetiology of breast cancer in women aged 50 to 74 years.

It is unclear whether age at menarche is causally involved in breast-cancer aetiology, or serves a correlate of other early-life exposures. Other aspects of reproductive life, including cycle length and regularity, climacteric symptoms, reproductive history and oral contraceptive use, are also incompletely investigated. We examined these issues in a population-based case-control study, including 3,016 women aged 50 to 74 years with invasive breast cancer, and 3,263 controls of similar age. Mailed questionnaires and telephone interviews were used to collect information on menstrual and reproductive characteristics as well as use of oral contraceptives. We found a statistically significant negative association between increasing age at menarche and breast-cancer risk in women born before 1925 but not after. Length of the menstrual cycle at age 30 seemed to be adversely related to breast-cancer risk, with OR for women with cycle lengths < 24 days and > 30 days being 0.76 and 1.18, as compared with women with a cycle length of 28 days. There was a strong trend of decreasing breast-cancer risk with increasing parity (OR per borne child 0.85, 95% CI 0.80-0.90). Lactation, menopausal symptoms or past use of oral contraceptives did not appear associated with breast-cancer risk. Our findings provide some evidence of a role of environmental correlates of early menarche in breast-cancer aetiology, and underline the importance of childbirth, especially early in life, in the prevention of breast cancer. Our data are not readily compatible with an important influence of former oral contraceptive use on post-menopausal breast-cancer risk.