Placental weight and breast cancer survival in young women.

A growing body of evidence indicates that reproductive history influences survival in breast cancer, especially among women diagnosed during or shortly after a pregnancy. However, little is known about the underlying mechanisms. We hypothesized that increasing placental weight, as an indirect marker of exposure to elevated hormone levels during pregnancy, would be associated with reduced survival in breast cancer. A cohort of 1873 women with at least one pregnancy after January 1st, 1973, and a subsequent breast cancer diagnosis before the end of 1991 were followed up for death or emigration through 2006. Information on placental weight and potential confounding factors were collected from medical records and from nationwide registers, which resulted in data on placental weight in the most recent pregnancy before diagnosis for 1,057 cases. For each 100-gram increase in placental weight, the adjusted hazard ratio of death was 1.09 [95% confidence interval (CI), 0.99-1.19]. The association was stronger among primiparous women (adjusted hazard ratio, 1.26; 95% CI, 1.09-1.47), and among women diagnosed during pregnancy or within 2 years from last birth (adjusted hazard ratio, 1.30; 95% CI, 1.06-1.59). Increasing placental weight is associated with reduced breast cancer survival. These findings are consistent with the hypothesis that the reduced survival in breast cancer among women with a recent childbirth is linked to pregnancy hormone exposure.