Maternal hyperglycemia may result in fetal hyperinsulinemia and asymmetric septal hypertrophy, macrosomia, and hypoglycemia in infants of diabetic mothers. We monitored glycosylated hemoglobin levels in 61 pregnant diabetic women each trimester as an index of maternal glycemic control and did serial fetal echocardiograms starting at 18 weeks of gestation. At delivery, cord blood C-peptide levels were obtained as an index of fetal hyperinsulinemia. Infants were assessed for hypoglycemia, macrosomia and septal thickening by echocardiography. Nineteen of the 61 infants (31%) had septal hypertrophy, were heavier, and had higher cord blood C-peptide levels and lower serum glucose levels than unaffected infants. Maternal glycosylated hemoglobin levels were higher during the third trimester in mothers of affected infants. Our data support a possible relationship between third-trimester maternal hyperglycemia and neonatal asymmetric septal hypertrophy, macrosomia, and hypoglycemia.