Objective: This study aimed to investigate the effect of magnesium sulfate treatment on fetal cardiac function during seizure prophylaxis of pregnant women diagnosed with severe preeclampsia and eclampsia. Fetal cardiac function was evaluated with the modified myocardial performance index (mod-MPI) before and after prophylaxis. Methods: A total of 50 pregnant women diagnosed with severe preeclampsia or eclampsia were involved. The characteristics of the participants, medical and obstetric history, ultrasound and laboratory findings were recorded. Fetal mod-MPI was obtained before and after magnesium treatment. Data were analyzed with appropriate statistical tests and p < 0.05 was considered to indicate statistical significance . Results: The mean maternal and gestational ages of pregnant women were 31.7 ± 5.9 and 32.4 ± 4.5. Mean mod-MPI before and after prophylaxis were 0.41 ± 0.18 and 0.49 ± 0.09, respectively (p<0.05). Conclusion: There was a significant increase in fetal mod-MPI after magnesium prophylaxis was given to severe preeclampctic/eclamptic women. The higher the mod-MPI, the worse the fetal cardiac function. This might have been the result of magnesium leading to an increased isovolumetric relaxation time. Key words: Preeclampsia, magnesium sulfate, myocardial performance index, eclampsia, fetus