Many new mothers do not reach their breastfeeding goals. Breastfeeding self-efficacy is a modifiable determinant influenced by prior and new breastfeeding experiences. More knowledge about factors associated with early breastfeeding experiences and breastfeeding self-efficacy would allow us to qualify breastfeeding counselling and increase breastfeeding duration. This study aimed to identify prevalence and factors associated with early negative breastfeeding experience, low breastfeeding self-efficacy in the first week postpartum, and drop in self-efficacy from late pregnancy to early postpartum period. A prospective longitudinal study was performed in Denmark from 2013 to 2014, including 2, 804 mothers. Results showed that 1 week postpartum almost 10% of mothers had negative breastfeeding experiences, 36% had low breastfeeding self-efficacy, and 26% drop in self-efficacy from pregnancy. Negative breastfeeding experiences were significantly associated with epidural analgesia, interrupted skin-to-skin contact immediately postpartum, short previous breastfeeding duration, and lacking social support. Low breastfeeding self-efficacy was associated with low breastfeeding intention, short previous breastfeeding duration, and negative breastfeeding experiences in the first week postpartum. Finally, significant associations of drop in breastfeeding self-efficacy from late pregnancy were no or short education, early negative breastfeeding experiences, prior short breastfeeding duration, and low general breastfeeding self-efficacy in pregnancy. Negative breastfeeding experiences in the first week postpartum is crucial for maternal breastfeeding self-efficacy 1 week following birth. It is important to identify and support mothers at risk of negative breastfeeding experiences in the first week following birth and address factors that might increase the probability of early successful breastfeeding experiences.