Events of reproductive life and the incidence of epithelial ovarian cancer

Abstract

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.

PIP: The association between factors of reproductive life and the occurrence of epithelial ovarian cancer were examined and anovulation and reduced gonadotrophin secretion were considered as mechanisms through which such factors might play their protective roles. All women with newly diagnosed epithelial ovarian cancer and who were resident in 6 counties of Washington and Utah during 1976 through 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 counties. To consider adequately the simultaneous effects of multiple relevant and possibly confounding variables, linear logistic regression techniques were used to analyze the data. Women with cancer reported fewer full-term pregnancies, fewer miscarriages, and less total time breastfeeding than controls. Cases in the Washington counties reported fewer exposures to combined oral contraceptive (OC) preparations. The difference between cases and controls was not apparent in the Utah data, possibly because of the low frequency of OC use among Utah residence and the small size of Utah samples. Obesity, defined as more than 20% excess weight at age 30 over the upper limit of ideal for a woman of a given height and medium frame was reported slightly more often by the cases than by the controls. Results obtained for reproduction variables appeared largely consistent with those of previous studies, in that factors associated with suppression of ovulation were generally protective. Histories of childbearing, miscarriages, lactation, and (in Washington) OC use were found to be associated with decreased risk of ovarian cancer. The estimated relative risks were, respectively, 0.88/pregnancy, 0.82/miscarriage, 0.79/year of lactation, and 0.89/year of OC. It was observed that the magnitudes off the diminished risks from these exposures substantially exceeded those which would have been expected solely on the basis of their inhibition of ovulation. 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 fails to reduce risk of ovarian cancer. Pregnancy, lactation, and OC use appear to offer some protection against the development of epithelial ovarian cancer, yet the reasons remain obscure.