Background: Preterm labor is frequency uterine contractions, pro- gressive effacement and dilation of the cervix prior to term gestation. Prevention and treatment of preterm labor is important because it is one of the most important causes of perinatal morbidity and mortali- ty. The etiology of preterm labor is poorly understood. Aim of the Work: to evaluate the influence of antenatally admin- istrated magnesium sulfate (MgSO4) given to women at risk of preterm birth on the cerebral blood flow and systemic hemodynamic in preterm infants. Patients and Methods: This was a prospective observational in- terventional study conducted from October 2016 to February 2017 on (40) patients undergoing preterm labor at AL Zahraa hospital Al Azhar university and New Cairo Hospital, after informed a written consent was taken from every patient after counseling them the pro- cedure of the study. Results: In our study we found that the best blood level of magne- sium sulfate at which contractions stopped (cut off) is (>4.18) mg/dl which equal (6.82) gm of administrated magnesium sulfate with sen- sitivity (94.74%) and specificity (100%), positive predictive value (PPV) was (100%) and negative predictive value (NPV) was (50%). Conclusion: Maternal magnesium sulphate (Mgso4) decreases the cerebral perfusion pressure and blood flow, and this likely protects the germinal matrix against the development of circulatory stress in the early post natal period. Magnesium sulphate achived the primary outcome (prevention of delivery for 48 hours, with uterine quies- cence) at cut-off value of (≥4.18) mg/dl which equal (6.82) gm with sensitivity (94.74%), specificity (100%), positive predictive value (100%) negative predictive value (50%).