Placental weight and breast cancer risk in young women: a registry-based cohort study from Norway.

BACKGROUND Pregnancy has a short-term risk-increasing effect on breast cancer that may be attributed to growth-promoting effects of pregnancy hormones on prevalent but undetected tumors. Results of two previous studies suggested that placental weight may be positively associated with breast cancer risk. METHODS In a cohort of 338,051 women followed from 1999 to 2008, on the basis of data linkage between the Medical Birth Registry of Norway and the Cancer Registry of Norway, we assessed whether placental weight in a woman's most recent pregnancy was related to breast cancer risk during the first years following pregnancy. RESULTS During follow-up (median, 6.0 years; interquartile range, 3.0-8.3 years), 648 women were diagnosed with breast cancer at a mean age of 38.4 years (standard deviation, 5.3 years). Placental weight in the most recent pregnancy was not associated with breast cancer risk: the hazard ratio per 100-gram increase in placental weight was 1.03 [95% confidence interval, 0.96-1.10]. There was a similar lack of association related to mean placental weight across pregnancies and to placental weight associated with the first birth. CONCLUSION We could not confirm previous reports that women who develop large placentas are at increased risk of breast cancer. IMPACT The epidemiologic support for an association of placental weight with breast cancer risk remains inconclusive. More research is needed to identify factors that influence breast cancer risk in young women.