REGULARITY AND LENGTH OF MENSTRUAL CYCLES IN WOMEN AGED 41-46 IN RELATION TO BREAST-CANCER RISK - RESULTS FROM THE DOM-PROJECT

The effect of regularity and length of the menstrual cycle on breast c ancer risk was studied prospectively in 78 cases and 383 age-matched c ontrols who participated in a breast cancer screening programme, the D OM-project, in Utrecht, the Netherlands. Before entering the screening programme when they were aged 41-46, the women kept a menstrual calen dar during at least three consecutive cycles. Cycles were considered t o be irregular if any of three cycles was shorter than 21 days or long er than 35 days and/or if variation between cycle lengths was more tha n five days. Women with irregular cycles had a significantly reduced r isk of breast cancer (odds ratio = 0.44; 95% confidence interval 0.22- 0.86) after adjustment for age at menarche, age at first birth, parity , Quetelet's index and family history of breast cancer. Among regularl y menstruating women, long cycles (28 days or more) were not significa ntly associated with increased risk of breast cancer (odds ratio 1.17; 95% confidence interval 0.66-2.09). To the extent that irregular mens trual cycles reflect anovulatory cycles, our findings support the hypo thesis that the cumulative number of regular ovulatory cycles increase s breast cancer risk.