Objective: To evaluate the alterations of fetal cardiac sizes and function in gestational diabetes mellitus(GDM) pregnancies under different maternal glucose control. Methods: In two tertiary obstetrics hospitals of Shanghai, 44 GDM mothers received 78 fetal echocardiography examinations in the study period and were divided into good(DM1) and poor(DM2) glucose control groups due to their glucose levels at examination. 70 uncomplicated mothers were as control. Parameters of cardiac sizes and function were measured and analyzed by gestational ages subgroups (<28, 28-34 and ≥34weeks). Results: In GDM fetuses the ventricular walls, especial interventricular septum were thicker than control, DM1 group more significantly than DM2 group. Peak flow velocities of aorta and pulmonary artery in GDM fetuses increased since <28 and 28-34weeks subgroups respectively, and right/left cardiac outputs ratio decreased slightly during gestation unlike its increase in control group. Left atrial shortening fraction and tricuspid E/A ratio showed smaller in GDM fetuses than control since ≥34 and 28-34weeks subgroups respectively with no significant differences between DM1 and DM2 group. Left and right ventricular Tei index were higher than control since 28-34 and <28weeks subgroups respectively in DM1 fetuses, but significantly increased till ≥34weeks subgroup in DM2 fetuses. Conclusion: Fetal cardiac sizes and function changed in GDM pregnancy, mainly the thickening interventricular septum, increasing blood flow and left cardiac outputs, decreased diastolic and global ventricular function. Tei index changed earlier and right ventricle was influenced earlier than left ventricle. More earlier and significant changes were observed in fetuses of poor maternal glucose control.