Combined effect of family history and reproductive factors on breast cancer risk.

This study evaluated the combined effect of a family history of breast cancer and each of three reproductive factors on breast cancer risk in the sisters of 404 breast cancer patients. The patients had a family history of breast cancer in a sister and mother or in two sisters. Risks were highest in sisters who had menarche at an early age or who first gave birth at a late age whether the patient had unilateral or bilateral disease, and with low parity only when the patient had bilateral disease. The bilateral group may have included a higher fraction of hereditary forms of breast cancer than the unilateral group. Because similar findings have been reported previously for breast cancer patients in general, the current results suggest that these reproductive factors have an effect on breast cancer risk independent of a family history of the disease. It is important, therefore, that the combined effects of these risk factors be taken into account when counseling women about their breast cancer risks.