Mortality from ovarian cancer among women with many children.

The exposure range for number of children as a risk factor for ovarian cancer has been severely restricted in most studies. This study is the first to describe the effect of many children, i.e. six or more, on the risk for death from ovarian cancer. The association between number of children in the current marriage given at the Norwegian Census in 1970 and mortality from ovarian cancer was investigated in a prospective study of 800,814 women followed over 15 years. Among postmenopausal women a strong negative trend between number of children and mortality from ovarian cancer was found; women with 8-11 children had a relative risk (RR) of 0.30 (95% confidence interval (CI): 0.17-0.54). For postmenopausal women the rate differences according to parity were constant across age strata indicating an additive model. No significant effect was found for age at first birth or social class. The study indicates that low parity could be the major risk factor for death from postmenopausal ovarian cancer.