Background: Breast-milk expression is widely practiced by Amer-ican mothers, but little is known about who expresses milk, how expression affects breastfeeding, or whether overweight or obese women, who have less breastfeeding success than do normal-weight women, express milk differently. Objectives: We investigated 1) whether breast-milk expression be-havior differed by body mass index (BMI; in kg/m 2) category and 2) whether the different breastfeeding behaviors of overweight (BMI: 25 and ,30) and obese (BMI: 30) women resulted in different breastfeeding outcomes. Design: The subjects (n = 2288) provided information on BMI and breast-milk production, feeding, and expression in mail-in question-naires as part of the Infant Feeding Practices Study II. Longitudinal and cross-sectional data were analyzed by using regression proce-dures adjusted for confounding. Results: Women of different BMI categories overall did not differ in whether, when, or why they expressed breast milk. Before 2 mo postpartum, however, obese women were more likely (P = 0.04, unadjusted) to try milk expression and were less likely (P = 0.01, unadjusted) to express milk successfully. In addition, overweight or obesity was associated (P , 0.03, unadjusted) with a shorter dura-tion of breast-milk production only in women who never expressed milk. In overweight or obese women, those who ever expressed milk had longer durations of breastfeeding (P , 0.003, unadjusted) than did those who never expressed milk. Conclusions: Breast-milk expression behaviors may differ by mater-nal BMI category only in the early postpartum period. In addition, breast-milk expression may reduce differences between BMI catego-ries in the duration of breastfeeding and support longer durations of breastfeeding.
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