We investigated cardiac function in 67 fetuses of gestational diabetic mothers (FGDMs) and 122 fetuses of healthy mothers between 24 and 36 weeks of gestation. Cardiac functions were evaluated by M-mode, pulsed-wave, and tissue Doppler echocardiography. Fetal echocardiograms were performed at 24, 28, 32, and 36 weeks of gestation. Glycated hemoglobin (HbA1c) levels were obtained from all pregnant women at 24 weeks of gestation. The mean age of diabetic pregnant women was significantly greater than that of controls. Serum HbA1c values of both groups were within normal limits, but they were significantly greater in the diabetic group (p = 0.003). The increase in peak aortic and pulmonary artery velocities were greater in FGDM (p < 0.001). No pathological interventricular septal (IVS) hypertrophy was observed. There was a significant increase in IVS thickness in FGDM compared with controls, which was more prominent at the end of the third trimester (p < 0.001). During the course of pregnancy, mitral E-wave (p < 0.001), A-wave (p = 0.007), tricuspid E-wave (p < 0.001) and A-wave (p = 0.002) velocities were greater in FGDM. The increases in mitral E/A and tricuspid E/A ratios were lower in FGDM with advancing gestation. The E (a)-wave (p = 0.02), A (a)-wave (p = 0.04), and S (a)-wave (p < 0.001) velocities of the right-ventricular (RV) free wall and the E (a) (p = 0.02) and A (a) (p = 0.01) velocities of the left-ventricle (LV) posterior wall were greater in FGDM during the course of pregnancy. The E (a)/A (a) ratio of the RV posterior wall was greater in FGDM with advancing gestation (p < 0.03). LV and RV E/E (a) ratios were similar in both groups. The LV myocardial performance index measured by pulsed-wave Doppler was greater in FGDM (p < 0.001). We detected diastolic dysfunction in FGDM. The data suggest that gestational diabetes mellitus may impair ventricular diastolic functions without causing pathological fetal myocardial hypertrophy. We detected subclinical diastolic dysfunction using both pulsed-wave and tissue Doppler imaging in FGDM.