Multiple births and maternal risk of breast cancer.

Data from the Cancer and Steroid Hormone Study, a large nationwide population-based case-control study conducted in the United States in 1980-1982, were analyzed to investigate whether pregnancies ending in a multiple birth affect the risk of subsequent breast cancer. The cases were 3,918 parous women who were aged 20-54 years and newly diagnosed with breast cancer; controls were 4,047 parous women selected randomly from the same geographic areas as the cases. Multiple births were reported by 118 cases and 161 controls. After adjustment for other reproductive variables, having a multiple last birth was found to be protective against breast cancer (odds ratio (OR) = 0.60, 95% confidence interval (CI) 0.43-0.85), whereas having a multiple birth prior to the last birth was not (OR = 1.11, 95% CI 0.79-1.57). To the authors' knowledge, this study is the first investigation to report such a protective effect, and thus the finding warrants replication. One mechanism that might account for the effect involves the increased output of alpha-fetoprotein by multiple fetal livers.