The relationship between lifelong menstrual pattern and the risk of breast cancer has been analyzed using data from a large case-control study conducted since 1983 in the greater Milan area including 3,037 cases, aged 74 years or less, with histologically confirmed breast cancer and 2,569 control subjects admitted for acute nongynecological, nonneoplastic nonhormone-related conditions to the same network of hospitals where cases had been identified. Compared with women reporting lifelong regular cycles, those reporting irregularities were at significantly reduced risk of breast cancer, relative risk estimate being 0.6, with 95% confidence interval (CI) ranging from 0.5 to 0.8. This protection was restricted to women reporting totally irregular menstrual cycles; compared to women reporting menstrual cycles lasting 25 days or less, the estimated relative risks were 1.0 and 1.2 in those reporting cycles lasting 26-30 days or 31 days or more, respectively, but 0.6 (95% CI 0.5-0.8) for those reporting totally irregular cycles. The effect of irregular menstrual cycles was similar in different strata of age and main risk factors for breast cancer.
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