This study was designed to determine which factors characterize mothers who expressed their milk by the end of 4 weeks postpartum as well as the duration for which they continued any breastmilk feeding.This prospective longitudinal cohort study was conducted with women who donated their milk and clinical data to the Cincinnati Children's Research Human Milk Bank (Cincinnati, OH). We examined the characteristics and length of breastmilk feeding of mothers who expressed their milk within the first month postpartum compared with those mothers who only fed directly at the breast.By the end of the first 4 weeks postpartum, 63% (37 of 59) of the mothers had begun milk expression. Predictors of milk expression by 1 month were planned work by 6 months, lower infant birth weight, and higher maternal body mass index. Milk expression by 4 weeks did not significantly influence duration of breastmilk feeding.Breastmilk expression in this cohort was common even within the first month postpartum before mothers in the United States typically go back to work. "Breastfeeding" classification needs to be updated to include options for breastmilk expression so the appropriate study of health outcomes related to this practice can be determined.