Background: Offspring from women with gestational diabetes mellitus (GDM) are at risk for later overweight, and the aim of treatment regimens is to normalize their prognosis. While the general concept is that breastfeeding is protective and should be promoted, some studies report increased levels of insulin and glucose in breast milk of women with diabetes, possibly increasing risks to the children. Previous studies may have low retention rates or mix GDM and pre-GDM, and often knowledge of confounders like maternal body mass index (BMI), level of hyperglycemia and feeding patterns is lacking. Data on breastfeeding rates, growth patterns and their associations are important to optimize future strategies among offspring from women with GDM managed by diet. Methods: Based on 10.730 births, a cohort of 131 singletons of Danish women with GDM managed by diet was defined. Data on feeding patterns, offspring length, weight and head circumference were obtained at the initial admission and from examinations by the general practitioner at five weeks and at five months postpartum. Breastfeeding rates were described in relation to neonatal and maternal characteristics and compared to national rates, while anthropometric data were compared to reference standards. The association between breastfeeding and offspring growth was analysed with and without correcting for confounding. Results: More than 99% of the cohort contributed to anthropometric data, while data on feeding patterns were available for 96-98%. Of mothers, 8% did not initiate breastfeeding and the rate of fully breastfeeding at five weeks and at five months of age were 61% and 18%, respectively, which is considerably lower than generally reported in Denmark. Lowest breastfeeding rates were seen following prelabour Caesarean delivery. Complementary feeding was introduced earlier than recommended among 11%. At the age of five weeks and at five months, children had grown longer and had lower BMI than expected from Danish and World Health Organization references. In the study periods, breastfeeding was significantly associated with lower BMI. Conclusion: Despite lower breastfeeding rates than normally reported in Denmark, offspring BMI at the age of five months were low. Still new initiatives to promote breastfeeding among Danish women with GDM should be considered.