ObjectivesHigh body mass index (BMI) often predicts truncated breastfeeding, although why is unclear. We test a proposed mediating role of body concerns on breastfeeding initiation and child's age at weaning using longitudinal data for 55,522 mothers from the Norwegian Mother and Child Cohort Study (MoBa).MethodsA linear regression‐based mediation analysis with bootstrapping estimates the indirect effects of BMI on breastfeeding decisions (ever‐initiation of breastfeeding, child's age at weaning, and duration of any breastfeeding beyond six months) through the variables of concern around prepregnancy weight and weight gains due to pregnancy.ResultsContrary to prediction, Norwegian mothers with greater prepregnancy weight concerns had a higher likelihood of initiating breastfeeding. Concerns about weight gain during pregnancy, however, predicted earlier weaning. This relationship was the same for higher and lower BMI mothers.ConclusionIn this very large sample, body image affects some breastfeeding decisions. However, this effect is independent of mother's body size.