A new study has reported that neither high pre-pregnancy body mass index (BMI) nor excessive weight gain during pregnancy are associated with subsequent risk of breast cancer in daughters. Previous studies have reported that high birth weight daughters have increased risk of breast cancer. However, the mechanisms underlying this association have not been established. The study included 2,623 women in the Nurses' Mothers study. A total of 814 mothers of nurses with breast cancer and 1,809 controls (mothers of nurses without breast cancer) provided data concerning pre-pregnancy height and weight, weight gained during pregnancy, and other aspects of their pregnancies. The average pre-pregnancy BMI was 21 kg/m2 in study participants; average weight gain during pregnancy was 23 lb.

Mothers' pre-pregnancy BMI was not found to be associated with the daughters' breast cancer risk during adulthood, when comparing the daughters of mothers who had had pre-pregnancy BMI of at least 30 to daughters of those with BMI under 20. Weight gain during pregnancy was also not found to be associated with daughters' breast cancer risk when comparing nurses whose mothers had gained 20 to 29 lb to daughters of those who had gained less than 10 lb. Nurses whose mothers had gained at least 40 lb also did not have heightened risk of breast cancer in adulthood. The authors conclude that the established association between birth weight and breast cancer risk is likely due to factors independent of mothers' pre-pregnancy BMI or weight gain during pregnancy. The authors comment that because BMIs and pregnancy weight gains are higher today, the study does not rule out associations for very high pre-pregnancy BMIs or pregnancy weight gains.

Please see our article on how to protect our daughters from breast cancer during the prenatal period and infancy for more information.