The aim of this study was to psychometrically test the Adaptation to the Late Preterm Infant when Breastfeeding Scale (ALPIBS) and also to test how a mother's self-efficacy predicts adaptation to a late preterm infant when breastfeeding. This study had a longitudinal and prospective design, and data collection was consecutive. Mothers (n = 105) with infants born between 340/7 and 366/7 weeks were recruited from a neonatal intensive care unit or a maternity unit. The ALPIBS was developed using exploratory factor analysis, and the association between breastfeeding self-efficacy and ALPIBS score was examined using linear regression analysis. The Breastfeeding Self-Efficacy Scale-Short Form instrument was used to measure self-efficacy in breastfeeding. A higher degree of self-efficacy was significantly associated with a higher degree of adaptation to the late preterm infant's breastfeeding behavior (P < .001). We identified 4 separate underlying factors measured by 11 items in the ALPIBS: (A) breastfeeding is a stressful event; (B) the infant should breastfeed as often as he or she wants; (C) a mother has to breastfeed to be a good mother; and (D) it is important to ensure control over the infant's feeding behavior. There is a link between self-efficacy and ALPIBS score, and self-efficacy is a modifiable factor that influences breastfeeding.