Objective: To evaluate the fetal cardiac function by modified myocardial performance index (Mod-MPI) in pregnant diabetic patients. Methods: Modified myocardial performance index was measured in fetuses of patients with diabetes (DM) between 30 and 40 weeks of gestation. The isovolumetric contraction time, isovolumetric relaxation time and ejection time were measured. Mod-MPI was calculated as (isovolumetric contraction time+isovolumetric relaxation time)/ejection time. Results were analyzed by using one-way analysis of variance (post hoc Bonferroni correction), Chi-square and Student's test. Results: Modified myocardial performance index values of fetuses from DM mothers were significantly higher than controls (0.43 vs 0.37, P<0.0001), and it was not different between gestational versus pregestational DM (0.42 vs 0.45, P=0.18). Mod-MPI was also higher in the presence of polyhydramnios (0.49 vs 0.41, P<0.0001), insulin use (0.46 vs 0.40, P<0.05), and large for gestational age fetuses (0.49 vs 0.40, P<0.0001). There were no significant differences in Mod-MPI between newborns with versus without neonatal complications such as hypoglycemia or polycythemia. Conclusions: In fetuses of DM mothers, evaluation of the Mod-MPI identifies those with worse maternal disease and large fetal size.