Fetal macrosomia is commonly associated with gestational diabetes mellitus (GDM) which may lead to various complications. It has been suggested that some other metabolites apart from maternal hyperglycemia are responsible for the genesis of macrosomia. Lipid metabolism changes in GDM patients having macrosomic fetuses were studied. A lipid tolerance test (10% Lipovenous solution) was performed in 14 GDM. Pre- and post-infusion plasma lipid levels and their elimination rates were measured and compared to the ones of 8 non diabetic control pregnant women. HbA1c, basal glucose and triglyceride levels were found to be higher in GDM group and significantly higher levels of triglycerides persisted throughout the infusion. FFA, glycerol and phospholipid levels increased following infusion in both groups without significant differences. Glucose, C-peptide and insulin levels remained unchanged after the infusion. Increased basal triglycerides with slowed triglyceride metabolism may be responsible for the fetal macrosomia in mild GDM patients whose fasting blood glucose are below 105 mg/dl. A better metabolic control that provides plasma lipid regulation as well as glucose control may forestall the occurrence of fetal macrosomia.