This study aimed to determine whether poor glycemic control in early pregnancy is associated with an increased risk of congenital heart disease (CHD) for infants of women with preexisting diabetes. A retrospective review examined two tertiary care centers of diabetic pregnancies that recorded early hemoglobin A1c (HbA1c) values (<20 weeks). The incidence of prenatally diagnosed CHD was calculated and stratified by HbA1c level. Poor glycemic control was defined as an HbA1c level of 8.5 % or higher. Fetal echocardiography was used to identify fetuses that resulted in infants with suspected CHD. Neonatal echocardiograms and pathology reports were reviewed for confirmation of the diagnosis. Of 535 patients, 30 (5.6 %) delivered an infant with confirmed CHD. Among the patients with poor glycemic control, 8.3 % (n = 17) delivered an infant with CHD, whereas 3.9 % (n = 13) of those with an HbA1c level lower than 8.5 % delivered an infant with CHD (p = 0.03). Poor glycemic control in early pregnancy is associated with an increased risk of CHD in offspring. The incidence of CHD in patients with adequate glycemic control still is sufficiently high to justify routine fetal echocardiography for all gravidas with preexisting diabetes regardless of HbA1c level.