A PROSPECTIVE-STUDY OF REPRODUCTIVE FACTORS AND RISK OF EPITHELIAL OVARIAN-CANCER

Background. Parity and long term use of oral contraceptives have been associated consistently with a decreased risk of ovarian cancer. Howev er, previous reports of the relationship of other reproductive factors (time since first use or last use of oral contraceptives, age at mena rche or menopause, age at first birth) with ovarian cancer have been i nconsistent. Methods. The authors studied these relationships in the N urses' Health Study, a prospective cohort study of 121,700 female regi stered nurses aged 30-55 years in 1976 when the study began. From 1976 to 1988, 260 cases of confirmed epithelial ovarian cancer occurred am ong 1.2 million person-years of follow-up. Results. A statistically si gnificant inverse association was observed between parity and ovarian cancer risk (relative risk [RR] = 0.84; 95% confidence interval [CI]= 0.77-0.91 per pregnancy); age at first birth was not associated indepe ndently with risk. In age-adjusted analyses, a significant inverse ass ociation was noted between long term use of oral contraceptives and ov arian cancer, which was no longer significant after controlling for ot her ovarian cancer risk factors (RR with greater than or equal to 5 ye ars' use: 0.65; 95% CI = 0.40-1.05). After control for duration of use , a weak nonsignificant inverse association was observed with time sin ce first oral contraceptive use and no independent effect of time sinc e last use. Neither age at menarche nor age at menopause was associate d significantly with ovarian cancer risk. Conclusions. In this large p rospective study, parity was the only reproductive factor that had a s ubstantial independent association with ovarian cancer. Long term oral contraceptive use also appeared to have an inverse relationship with ovarian cancer, although this association was of borderline significan ce (P = 0.11) after adjustment for other risk factors.