Abstract Objectives: Describing preterm breast milk evolution and composition according to gestational age (GA) and postnatal age (PNA) in a cohort of mothers cared for in an ambulatory Kangaroo Mother Care Program (KMCP) in a developing country. Methods: A cohort involving 113 mothers who delivered ‘healthy’ preterms adequate for GA was assembled. Mothers received intensive breastfeeding support before discharge. Samples of both fore‐ and hind milk were obtained at entry into KMCP and weekly thereafter, until term. Composition was described according to PNA and postconceptional age (PCA). Results: Protein concentration varied inversely with both PCA and PNA. Fat concentration was consistently higher in hind milk than in fore milk samples of the same feed. Lactose increased steadily with PCA. Calcium/phosphorus ratios were stable, close to 2:1 and content of both was similar in samples of different PCA and PNA. Conclusion: Minerals concentration could be inadequate for preterms. Protein concentration decreases steadily to mature milk levels by the third week of PNA, regardless of birth GA. Therefore, from the third week of PNA onwards, protein content could be insufficient to satisfy needs of preterms born at 32 weeks or less. Feeding hind milk could increase caloric density and fat intake to better meet preterms’ nutritional needs.