The aim of this study was to evaluate the impact of pregestational diabetes mellitus (DM) on fetal cardiac function two-dimensional parameters using spectral and tissue Doppler. Pregnant women between 20 and 36 + 6 weeks gestation were divided into three groups: controls, type I DM, and type II DM. The right ventricle (RV) and left ventricle (LV) annular velocity peaks were measured using spectral (E, A) and tissue (E′, A′, S′) Doppler. The myocardial performance index was calculated as (isovolumetric contraction time [ICT] + isovolumetric relaxation time [IRT])/ejection time using tissue (MPI′) and the spectral Doppler (MPI). A general linear model, with fetal heart rate as a covariant, was used to evaluate the effect of DM on the fetal heart function assessment parameters. To assess the association of type I and II DM with adverse perinatal outcomes, Fisher’s exact test was used. A receiver operating characteristic curve was used to determine the best cutoff for fetal cardiac function assessment parameters to identify the neonatal composite outcomes. The sample comprised 179 pregnant women. DM had significant effect on RV and LV A peak velocities (p = 0.026 and p = 0.011, respectively). LV ICT (p < 0.001) and LV MPI (p < 0.001) were significantly affected by maternal DM. Fetuses from pregnant women with type II DM showed significantly higher LV MPI (0.492 vs. 0.459, p = 0.006) and RV S′ (7.2 vs. 6.44 cm/s, p = 0.024) than controls. Fetuses from type I DM pregnant women showed increase in cardiac parameters that evaluated the LV and RV diastolic function (LV IRT′ p < 0.001 and RV MPI′ p = 0.044). Type I and II DM were associated with adverse perinatal outcomes: neonatal intensive care unit stay (p < 0.0001), macrosomia (p < 0.0001), hyperbilirubinemia (p < 0.0001), and hypoglycemia (p < 0.0001). The LV MPI′ showed significant but moderate sensitivity in identifying the composite neonatal outcomes (AUC: 0.709, 95% CI 0.629–0.780, p < 0.001). Tissue Doppler and MPI parameters can be useful to detect subclinical cardiac dysfunction in the fetal heart of pregestational DM pregnant women.