Investigating the association of lactation history and postmenopausal breast cancer risk in the Women's Health Initiative.

Prolonged lactation (≥24 mo) has been associated with reduced breast cancer risk. This research examined this association in postmenopausal women in the Women's Health Initiative (WHI) Hormone Trial (HT) and Observational Study (OS). This retrospective cohort analysis included 69,358 predominantly overweight (65.4%), white (83.2%) postmenopausal women without breast cancer. Women in the HT were randomized to 0.625 mg conjugated equine estrogen (CEE), 0.625 CEE + 2.5 mg medroxyprogesterone acetate (CEE/MPA), or placebo. OS participants had no restrictions on hormone use. Lactation history was assessed via WHI Reproductive History Questionnaire. Most women breastfed at least 1 mo (58.0%); 35.4% breastfed 1-2 children; and 6.5% stated having breastfed ≥24mo. Women in the HT-CEE who breastfed their first child between 20-24 yr of age demonstrated a nonsignificant decreased risk of breast cancer (HR: 0.62; 95% CI: 0.38, 1.01). OS participants who reported CEE/MPA hormone use and age of first breastfeeding ≥30 yr showed a significant increased risk of breast cancer (HR: 1.66; 95% CI: 1.14, 2.41). Risk was increased if age of last breastfeeding was ≥35yr (HR: 1.50; 95% CI: 1.05, 2.14). This research did not demonstrate a significantly decreased risk of postmenopausal breast cancer in women who breastfed for ≥24 mo during their lifetime.