Weight Gain in Pregnancy and Pre-menopausal Breast Cancer: a population-based case-control study
Background: The risk of breast cancer increases transiently after pregnancy and then falls to a lower level than for age-matched nulliparous women. Higher levels of estrogen are known as the primary reason. Several pregnancy characteristics are thought to be confounding the above link; however, except for the age at first pregnancy no other pregnancy-related factor has been proved to be significant. Specifically, it has not been established that an excessive weight gain in pregnancy is linked to the maternal risk of breast cancer.
Objectives: To identify independent influence of weight gain in pregnancy on the risk of pre-menopausal breast cancer following a delivery. We also examine the effect of demographic factors and other relevant pregnancy characteristics on the risk of breast cancer.
Methods: Our cohort consisted of 213,250 women who gave birth to a live neonate from January 1, 1994 to December 31, 2003. We performed a nested population-based case-control study on 126 Nevada patients with a first lifetime breast cancer diagnosed from January 1, 1995 through December 31, 2003, and 504 Nevada participants without breast cancer. The two groups were matched on age at the recorded delivery. Our dataset was a linkage between the Nevada Cancer Registry database for the years 1995-2008 and birth certificates from the Nevada State Health Division for the years of 1994-2008.
Results: The excessive weight gain in none of the categories was associated with increased risk of premenopausal breast cancer. After adjustment for other pregnancy characteristics the results remained unchanged. Women whose pregnancies progressed beyond 40 weeks of gestation were at significantly lower risk of developing breast cancer up to five years following delivery, compared to the women who delivered at 37-40 weeks of gestation (OR 0.33, 95% CI 0.11-0.93), after adjustment for confounding variables.
Conclusions: Our study did not show an association between excessive weight gain in pregnancy and the risk of pre-menopausal breast cancer. However, pregnancies that extended beyond 40 weeks of gestation were strongly associated with a lower likelihood of pre-menopausal breast cancer up to 5 years after delivery. Biological plausibility for this may relate to the fact that as pregnancy develops into more advanced stages, mammary cells have more time to complete differentiation and maturation; a process that started earlier in the third trimester. The results of this study will therefore contribute to the literature and may provide a basis for future studies.