Objective: First, to describe the prevalence of both full and partial breast-feeding during the first 6 months; second, to study the associations between selected health service-related factors and cessation of full breast-feeding at three time intervals. Design: Retrospective questionnaires, 6 months after birth. Setting: The Norwegian Mother and Child Cohort Study (MoBa). Subjects: In total, 29 621 women. Results: While 96?6 %, 94?0 %, 90?8 %, 86?9 %, 83?8 % and 80?0 % of the infants were breast-fed at 1, 2, 3, 4, 5 and 6 months, respectively, the corresponding proportions for full breast-feeding were 84?6 %, 79?1 %, 70?9 %, 44?0 %, 16?7 % and 2?1 %. An increased risk of cessation of full breast-feeding during the first month was associated with supplementation during the first week of life with water (relative risk (RR) 1?77; 95 % CI 1?52, 2?06), sugar water (RR 1?73; 95 % CI 1?49, 2?00) or formula (RR 5?99; 95 % CI 5?58, 6?42). An increased risk was also associated with Caesarean delivery (RR 1?08; 95 % CI 1?00, 1?16) and breast-feeding problems (RR 1?56; 95 % CI 1?45, 1?67). Between months 1 and 3, the risk of cessation of full breast-feeding remained elevated in the case of supplementation during the first week of life with water (RR 1?29; 95 % CI 1?14, 1?45), sugar water (RR 1?48; 95 % CI 1?34, 1?64) or formula (RR 1?18; 95 % CI 1?07, 1?29). The same applied to Caesarean delivery (RR 1?15; 95 % CI 1?06, 1?25). Conclusions: Supplementation during the first week, breast-feeding problems and Caesarean delivery are associated with early cessation of full breast-feeding. The results support a cautious approach to supplementation during the first week of life.