Breastfeeding can be challenging for new mothers. Women with high breastfeeding self-efficacy are more likely to breastfeed with confidence and are more likely to succeed in breastfeeding. The aim of this prospective longitudinal study was to test whether breastfeeding self-efficacy and breastfeeding experiences were related to trajectories of self-efficacy in the parenting domain during the transition to parenthood. A group of 1,091 primiparous women completed questionnaires at 32 weeks of gestation (maternal self-efficacy) and 3 months after giving birth (maternal self-efficacy, breastfeeding self-efficacy, and breastfeeding experiences). Only the women who started breastfeeding and completed questionnaires at both time points were included in the analyses (N = 817). High breastfeeding self-efficacy significantly predicted increased maternal self-efficacy through the transition to parenthood. A successful breastfeeding experience fully explained this effect. The mediating effect of positive breastfeeding experiences on the relationship of breastfeeding self-efficacy and the change of maternal self-efficacy during the transition to parenthood implies breastfeeding to be an early target to enhance maternal self-efficacy. By pointing out breastfeeding as a genuine challenge during prenatal maternity courses, it might prepare mothers for one of their first experiences of parenthood. As a result, successful breastfeeding could be a steppingstone from positive prenatal expectations to growing confidence as a parent.