Background: role of intravenous hydration is well documented for the treatment of oligohydramnios. The present study concentrated on the efficacy of intravenous hydration in cases of unexplained oligohydramnios.Aim of the study: this study aimed to evaluate the effect of intravenous maternal hydration (hypotonic and isotonic fluid) in pregnancies with third-trimester oligohydramnios on amniotic fluid index and uteroplacental perfusion and fetal blood flow. Patients and Methods: this study was conducted in the Department of Obstetrics and Gynecology, Faculty of Medicine, Al-Azhar University, Egypt, during the period from January 2017 to January 2018. A total of 60 pregnant women were included and parameters like age, parity, BMI, amniotic fluid index, urine specific gravity, hematocrit value, umbilical artery pulsatility index and uterine artery pulsatility index before and after maternal hydration were noted and compared. Results: intravenous maternal infusion of (1/2) normal saline (0.45%) at a rate of  1000 ml/hr for two hours has statistically difference in the mean amniotic fluid index ,in mean urine specific gravity, in mean umbilical artery pulsatility index and in mean uterine artery pulsatility index immediately after hydration,72 hr  and  one week after hydration (p<0.05). Conclusions: maternal intravenous hydration with hypotonic fluid can be used in the treatment of oligohydramnios