Pregnancy weight gain and premenopausal breast cancer risk

OBJECTIVE: To investigate whether excessive weight gain during pregnancy alters a woman's risk of developing premenopausal breast cancer. STUDY DESIGN: We conducted a nested case-control study within a cohort composed of 22,610 Finnish women (mean age, 40; SD 6.4) who responded to a questionnaire requesting information on breast cancer diagnosis and on adult and pregnancy weight gain as part of a study of a hormone-releasing intrauterine device. One hundred fourteen of these women reported having been diagnosed with breast cancer, and 98 were eligible for analysis. Four matched controls were selected for each case (n = 392). Mean age at the time the women were diagnosed with breast cancer was 41.3 years (SD 5.46) and at the time they returned the questionnaire, 46.7 years (SD 5.55). RESULTS: We found no evidence that pregnancy weight gain, unadjusted or adjusted for prepregnancy body weight or other covariates (educational status, age at menarche and first pregnancy, and family history of breast cancer), had an effect on breast cancer risk. However, pregnancy weight gain was linked to an inverse association between change in body mass index (BMI) during adult life and premenopausal breast cancer. Women who gained an excessive amount of weight during pregnancy (> 16 kg) were at a significantly reduced risk of developing premenopausal breast cancer (p = 0.043, OR 0.28, 95% CI 0.08-0.96) if their BMI increased >7 m/kg ^-2 after age 20. BMI increase was not associated with breast cancer risk in women whose pregnancy weight gain remained within the recommended range (< 16kg). CONCLUSION: Since our previous findings show that women who gain an excessive amount of weight during pregnancy are at an increased breast cancer risk after menopause, regardless of adult BMI, an excessive pregnancy weight gain may provide a short-term protective but cause a long-term breast cancer-promoting effect. It remains to be determined why BMI and pregnancy weight gain differently affect premenopausal and postmenopausal breast cancer risk.