AbstractObjectiveWe investigated the association between full breast-feeding up to 6 months as well as partial breast-feeding after 6 months and maternal weight retention at 6, 18 and 36 months after delivery in the Norwegian Mother and Child Cohort Study (MoBa), conducted by the Norwegian Institute of Public Health.DesignCohort study. Information on exposure and outcome was collected by questionnaire.SettingNorway.SubjectsWomen at 6 months (n 49 676), 18 months (n 27 187) and 36 months (n 17 343) postpartum.ResultsLonger duration of full breast-feeding as well as partial breast-feeding was significantly related to lower weight retention at 6 months. At 18 months full breast-feeding (0–6 months) and partial breast-feeding for 12–18 months were significantly related to lower weight retention. At 36 months only full breast-feeding (0–6 months) was significantly related to lower weight retention. For each additional month of full breast-feeding, maternal weight was lowered by 0·50 kg/month at 6 months, 0·10 kg/month at 18 months and 0·14 kg/month at 36 months (adjusted for pre-pregnant BMI, pregnancy weight gain, age and parity). Partial breast-feeding resulted in 0·25 kg/month lower maternal weight at 6 months. Interactions were found between household income and full breast-feeding in relation to weight retention at 6, 18 and 36 months, indicating most benefit among women with low income.ConclusionsThe present study supports the hypothesis that full breast-feeding contributes to lower postpartum weight retention and shows that the effect is maintained for as long as 3 years postpartum.