Events of reproductive life and the incidence of epithelial ovarian cancer.

Women resident in six counties of Washington and Utah for whom diagnoses of epithelial ovarian cancer were made during 1975-1979 were interviewed concerning their menstrual, reproductive and medical histories. For comparison, interviews were also obtained from a random sample of women living in the same countries. Logistic regression methods were used, and histories of childbearing, miscarriages, lactation, and (in Washington) usage of oral contraceptives were found to be associated with decreased risk of ovarian cancer; the estimated relative risks were, respectively, 0.88 per pregnancy (i.e., 0.88(2) for two pregnancies, etc.) (p = 0.016), 0.82 per miscarriage (p = 0.049), 0.79 per year of lactation (p = 0.034), and 0.89 per year of oral contraception (p = 0.009). In addition, it was observed that the magnitudes of the diminished risks from these exposures substantially exceeded those which would have been expected solely on the basis of their inhibition of ovulation (X2(5) = 21.5, p = 0.0006). On the other hand, the lack of association found between the occurrence of ovarian cancer and either total dose or total time of exposure to noncontraceptive estrogens, or with a history of usage of thyroid medications, suggests that periods of reduced pituitary gonadotrophin secretion fail to reduce risk of ovarian cancer. Thus, although pregnancy, lactation and oral contraception appear to offer some protection against the development of epithelial ovarian cancer, the reasons remain obscure.