Abstract. OBJECTIVE: To compare neonatal outcomes (including breastfeeding (BF) initiation) of 170 macrosomic IDM with that of 739 macrosomic nIDM. DESIGN/METHODS: Retrospective cohort investigation of all macrosomic infants born consecutively over a four-year period (2008–2011). Macrosomic (birth weight ≥4000 g) IDM included 100 infants whose mothers had gestational diabetes and 70 whose mothers had pregestational diabetes. RESULTS: IDM were more likely to be delivered by cesarean to obese women while nIDM were more likely to be delivered vaginally to younger women with a higher level of education. Ethnic distribution (60% white, 20% black, 10% Hispanic and 10% Asian or African) was similar in each group. Forty-nine percent of IDM and 7% of nIDM required NICU admission. Respiratory disorders (mainly TTNB) affected 21% of IDM and 3% of nIDM while hypoglycemia was observed in 36% of IDM and 15% of nIDM. Of the 35 IDM delivered vaginally, 10 were complicated by shoulder dystocia without injury. Conversely, 70 of the 458 nIDM delivered vaginally experienced shoulder dystocia that resulted in 6 limb fractures and 3 brachial plexus injuries. On arrival to labor and delivery, 75% of all women intended to BF; however, at the time of discharge, 65% of women with diabetes and 92% of those without diabetes who intended to BF had initiated BF. CONCLUSIONS: Both macrosomic IDM and macrosomic nIDM are at risk for significant morbidities. Macrosomic IDM carry a higher risk for NICU admissions, leading to maternal-infant separation, and lower BF initiation rates.