Reproductive factors and cancers of the breast and genital organs--are the different cancer sites similarly affected?

Relationships between reproductive factors and cancer of the breast and genital organs were investigated in a prospective study of 63,090 Norwegian women. During followup from 1961 through 1980 1565 cases of breast cancer, 422 cases of cancer of the corpus uteri, and 471 cases of ovarian cancer were diagnosed. High parity was associated with low risk of cancer of the breast, corpus uteri, and ovary in analyses with adjustment for age at first birth. Age at first birth and age at last birth were positively associated with risk of breast cancer and inversely associated with endometrial cancer, whereas no clear associations were observed with ovarian cancer. Results suggest that the effect of a pregnancy on cancer risk is mediated, at least in part, by different mechanisms for these three sites. Age at menarche was inversely and age at menopause positively related to risk of breast and endometrial cancer, whereas no association was observed for ovarian cancer. In analyses of squamous cell carcinoma of the cervix uteri, an association with reproductive factors was expected from the known relationships with sexual habits. However, this cannot account for the high risk observed in this study in women with early age at first birth.