The following variables were identified a priori as exposures that may modify the distribution of breastfeeding initiation: race, maternal education, maternal age at delivery, delivery type, birth weight, 3rd/4th degree laceration, labor induction, emergency cesarean delivery for fetal distress, use of forceps or vacuum, the need for a maternal blood transfusion post-partum, maternal infection/sepsis post-partum, and maternal hospital readmission for postpartum complications. In our analysis of within-woman differences in breastfeeding initiation across pregnancies, we also considered differences by birth order.