PARITY, CONTRACEPTION, INFERTILITY, AND THE RISK OF EPITHELIAL OVARIAN-CANCER

A case-control study of reproductive factors and cancer of the ovary w as conducted during 1989-1992 in metropolitan Toronto and nearby areas of Southern Ontario, Canada. In total, 450 women aged 35-79, years wi th histologically verified new primary epithelial ovarian cancers were interviewed concerning their reproductive histories. Over the same ti me period, 564 randomly selected population controls, frequency-matche d to the cases according to three 15-year age groups, were also interv iewed. Continuous unconditional logistic regression methods were used for analysis. It was found that childbearing and use of oral contracep tives were associated with significant decreasing trends in risk of ov arian cancer; the respective odds ratios were 0.78 for each full-term pregnancy (p < 10(-8)) and 0.92 for each year of use (p < 10(-6)). Hys terectomy was also associated with reduced risk, even after more than 20 years. Among parous women, infertility did not appear to affect ris k; for nulliparous women, some evidence of increased risk was present, although fertility problems were reported by only a small fraction of nulliparae. It is suggested that the relatively lower parity of cases as compared with controls may be due to voluntary choices for having fewer children.