MATERNAL RISK OF BREAST-CANCER FOLLOWING MULTIPLE BIRTHS - A NATIONWIDE STUDY IN SWEDEN

The association between multiple births and subsequent maternal breast cancer risk was explored in a nested case-control study in Sweden enc ompassing 19,368 parous women with breast cancer diagnosed up to age 6 5 years, and 100,459 parous controls. Among cases and controls, there were 329 and 2,031 women, respectively, with a history of at least one live multiple birth. Compared with singleton mothers, breast cancer r isk was 12 percent lower (odds ratio = 0.88, 95 percent confidence int erval = 0.78 - 0.99) in women who had had a multiple birth. After stra tification for age at diagnosis, evidence of a significant inverse ass ociation was found only in women aged 54 years or younger. Birth order of the multiple pregnancy had no apparent risk-modifying effect. Age at earliest multiple birth was unrelated to breast cancer risk. The in verse association between twinning and breast cancer risk may reflect protective physiological features of twin pregnancies. Further researc h is needed to investigate the role, if any, of increased levels of st eroid hormone-binding globulins in mothers of twins and the proposed i nhibitory effects of human chorionic gonadotropin and alpha-fetoprotei n, both of which are increased during multiple gestations, on breast c arcinogenesis, Breast feeding patterns in mothers of twins also may mo dify their risk of developing breast cancer.