MULTIPLE BIRTHS AND RISK OF BREAST-CANCER

Data from a large, multicenter, population-based case-control study we re analyzed to investigate the relation between multiple birth pregnan cies and subsequent breast-cancer risk in the gravidas. Women less tha n 75 years old who had breast cancer were identified from statewide tu mor registries in Wisconsin, western Massachusetts, Maine and New Hamp shire. Controls aged less than 65 years were selected randomly from li sts of licensed drivers, and controls aged between 65 and 74 were sele cted randomly from lists of Medicare beneficiaries. Information on rep roductive history and other factors was obtained by means of telephone interviews. After excluding nulliparous women, 5,880 case subjects an d 8,217 controls remained for analysis. Multiple birth pregnancies occ urred in 146 cases and 218 controls. Adjusted for the number of full-t erm pregnancies and other confounders, the odds ratio (OR) for any mul tiple birth pregnancy was 0.94 (95% confidence interval, 0.75 to 1.17) . The null association between multiple birth pregnancies and breast c ancer remained if the mother's first pregnancy or last pregnancy resul ted in a multiple birth. In addition, no trend in risk emerged for age at first multiple birth or time since last multiple birth. While othe r investigators have suggested biological mechanisms to explain both p rotective and detrimental effects of multiple births and subsequent de velopment of breast cancer, this study does not support either asserti on. (C) 1995 Wiley-Liss, Inc.