Background: Preterm labor is one of the commonest clinical events where traditional pregnancy can turn into a high risk situation for the mother as well as the fetus. Magnesium sulfate (MgSO₄), which is a calcium antagonist, has been used for over 30 years as short-term treatment (less than 48 h) for acute preterm labor. It acts by inhibiting voltage independent calcium channels in the myometrial cell surface. Extracellular magnesium suppresses calcium influx across cell membranes, whereas intracellular magnesium competes with calcium, there by inhibiting myosin light-chain kinase activity. Objective: To assess the effect of Mgso₄ as a tocolytic agent on Doppler parameters of fetal middle cerebral, umbilical and uterine arteries in women with threatened preterm labor. Patients and methods: A prospective observational study was conducted in Al Shohadaa Central Hospital in the period between September 2018 and September 2020. A total of 300 women with threatened preterm labor were included in the study. The study group underwent a Doppler ultrasound study to evaluate the flow velocity parameters resistibility index (RI), pulsatility index (PI) and systolic/ diastolic ratio (S/D) of fetal middle cerebral artery (MCA), umbilical artery (UA) and uterine artery (UtA) using the abdominal probe of the ultrasound device. Then immediately they were given an intravenous infusion of 4- 6 gm of magnesium sulphate as a loading dose then 1 gm / hour as Mgso₄ maintenance dose. The same Doppler Ultrasound parameters were recorded again after 24 hours Mgso₄ tocolysis to evaluate the effect of magnesium sulphate administration. Results: There was a no statistically significant reduction in RI, PI, SD ratio of the UA before 32 weeks gestation while a statistically significant reduction in these parameters after 32 weeks gestation following magnesium sulphate infusion. Regarding MCA Doppler parameters in our study, there was a statistically significant reduction in RI, PI, SD ratio after magnesium sulphate infusion. Regarding UtA Doppler parameters in our study, there was no statistically significant differences were found in uterine artery RI and S/D ratio following magnesium sulphate infusion, whereas uterine artery PI showed statistically significant reduction after magnesium sulphate infusion. Conclusion: Although the use of Mgso₄ as a tocolytic agent is controversial but it is valuable as neuroprotective agent and for treatment of eclamptic seizures. Its effect on the fetal blood flow has a very beneficial role till now. In this study it was found that it has significant effect on fetal MCA, UA and UtA Doppler parameters (RI, PI and S/D ratio). More studies with larger cohorts are needed to support these results.