The relationship between menstrual cycle patterns and the risk of breast disease was evaluated using data from a hospital-based case-control study of 288 women with benign breast disease (203 chronic cystic diseases and 85 benign tumours), 317 with breast cancer and 602 age-matched controls with a spectrum of acute conditions unrelated to any of the established or potential risk factors for breast disease. A lifelong irregular menstrual pattern [defined as frequent occurrence of menstrual-like episodes of bleeding less than 21 or more than 35 days apart) was negatively associated with the risk of benign breast lesions (relative risk, RR = 0.6, with 95% confidence interval = 0.4-1.0) and of breast cancer (RR = 0.4, with 95% confidence interval = 0.3-0.8]. This inverse association could not be explained by any of the identified potential confounding factors, including the major risk factors for breast disease. The findings of this study, showing that a lifelong history of irregular (and hence more likely anovular) cycles was less frequent among women with benign and malignant breast diseases, support the hypothesis that frequent ovular cycles might be more carcinogenic than anovular ones.
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