Women who gain 38 pounds or more while pregnant raise their risk of developing breast cancer after menopause by 40%.
Excess pregnancy weight gain doesn't increase breast cancer risk in pre-menopausal women, but it does increase risk for post-menopausal women.
The study found that women who gained 40 pounds or more during pregnancy raise their post-menopausal risk of developing breast cancer by 40%. In industrialized countries, about half of pregnant women gain weight within the recommended range, but about 36% gain too much. Excess pregnancy weight boosts breast cancer risk by about the same amount as being obese after menopause.
The study also found that women who fail to drop the added pounds after giving birth are most at risk. Pregnancy weight may impact cancer development because levels of the hormone estrogen, which are already raised during pregnancy, are amplified in fat tissue.