To study whether the fetal cardiac function evaluated with the modified myocardial performance index (Mod-MPI) is affected in fetuses from diabetic women, and to evaluate differences between gestational and pre-gestational diabetes. The study was performed in 54 fetuses from diabetic pregnant women and in 209 controls, between 30 and 40 weeks of gestation. The Mod-MPI was evaluated in the left cardiac chambers using the clicks of the valves to estimate the time periods. Diabetic mothers were classified as gestational diabetes (n = 43) and pre-gestational diabetes (n = 11). Fetal signs of diabetic descompensation were: polyhydramnios, defined as amniotic fluid index over the 90th percentile for GA and an increased estimated fetal weight < 95th centile for gestational age. Mod-MPI values of fetuses from diabetics mothers were significantly higher than controls (0.430 ± 0.06 vs. 0.366 ± 0.06, respectively: P < 0.0001). The gestational and pre-gestational group were not significantly different (0.421 ± 0.06 vs. 0.461 ± 0.07, respectively; P = 0.071). In fetuses with signs of metabolic descompensation there was a significant increment in the Mod-MPI (0.397 ± 0.042 vs. 0.493 ± 0.056, P < 0.0001). Evaluation of the Mod-MPI in fetuses of diabetic mothers might help to improve the clinical surveillance of these pregnancies.