Objective: To compare the relationship between umbilical artery, uterine artery and fetal middle cerebral artery using Doppler indices (resistive index, pulsatility index and the S / D) before and after use of magnesium sulfate in pregnant women with severe preeclampsia. Methods: A prospective cohort study was developed, in which 50 pregnant women with severe preeclampsia were selected, underwent Doppler sonography before and 20 minutes after intravenous administration of 6 g of magnesium sulfate. The variables studied were the relative rates of umbilical artery, uterine artery and fetal middle cerebral artery Doppler. The comparison of means between the two measurements (before and after) of each individual was performed.Results: There was statistically significant difference of means of umbilical artery, uterine artery and fetal middle cerebral artery Doppler indices before and after the magnesium sulfate. Conclusion: Intravenous administration of magnesium sulfate in pregnant women with severe preeclampsia resulted in a decrease in umbilical artery, uterine artery and fetal middle cerebral artery Doppler indices with reduced resistance to blood flow in these vessels.