Beyond ovulation: Oral contraceptives and epithelial ovarian cancer

In a case-control study in three Australian states that included 794 women with epithelial ovarian cancer and 853 community controls for whom we had a dequate contraceptive and reproductive histories, Re examined the effects o f oral contraceptive use after controlling for estimated number of ovulator y cycles. Other covariates included in the multiple logistic regression ana lysis were parity, smoking, and history of pelvic surgery. The protective e ffect of duration of oral contraceptive use appeared to be multiplicative, with a 7% decrease in relative risk per year [95% confidence interval (CI) = 4-9%], persisting beyond 15 years of exposure. Use for up to 1 year may h ave a greater effect than predicted (odds ratio = 0.57; 95% CI = 0.40-0.82) , whereas use before the first pregnancy may be additionally beneficial (od ds ratio = 0.95; 95% CI = 0.87-1.03, adjusted for overall duration of use). Better control for ovulatory life might attenuate these estimates somewhat . There was little evidence of waning protection with time since last expos ure or of extra benefit with early commencement of oral contraceptive use. We found no convincing evidence of effect modification in any factor examin ed or differences in effect among the three main histologic cancer types or between borderline and malignant tumors. Oral contraceptives may act by bo th suppressing ovulation and altering the tumor-promoting milieu.