Gestational Age and the Risk of Maternal Breast Cancer: A Population‐Based Case–Control Study

There is limited evidence that the gestational age at delivery may influence the risk of maternal breast cancer. While extreme prematurity has been suggested to increase the risk, there seems to be no study available so far that investigates the due effects of a late delivery. This research aimed to identify the impact of both preterm and late deliveries on the risk of maternal breast cancer within a period of 5 years after birth. Our dataset was created by linking data from the Nevada Cancer Registry database (1995–2008) and the birth certificates issued by the Nevada State Health Division (1994–2003). The study cohort consisted of 213,250 women who gave birth from January 1, 1994 to December 31, 2003. We performed a nested population‐based case – control study on 126 Nevada mothers with a first lifetime breast cancer diagnosed from January 1, 1995 through December 31, 2003, and 504 Nevada cancer‐free mothers. Women with pregnancies who progressed beyond 40 weeks of gestation were at a significantly lower risk of developing breast cancer for the 5‐year period following a delivery, when compared to women who delivered at 37–40 weeks of gestation (OR: 0.33, 95% CI: 0.11–0.92) in a multivariate model. Additional pregnancy characteristics did not significantly predict the risk of maternal breast cancer. Pregnancies that extended beyond 40 weeks of gestation were strongly associated with a lower likelihood of premenopausal breast cancer. Biological plausibility for this association may correspond to the fact that as pregnancy develops into more advanced stages, mammary cells have more time to attain complete differentiation and maturation; a process that starts early in the third trimester.