Background: Recurrent pregnancy loss (RPL) is three or more consecutive pregnancy losses before 20 weeks gestation. After a thorough evaluation, a cause is found in a little more than half of the cases and the rest remain unexplained. Theories suggest that impaired uterine perfusion may be a cause of recurrent unexplained pregnancy loss. Objective: To investigate the color Doppler sonographic parameters, using transvaginal imaging, of uterine artery and utero placental vasculature (retro chorionic vessels), and comparing them between women with no history of pregnancy loss and women with recurrent first trimester pregnancy loss of unexplained cause. Patients and Methods: This study was done in outpatient clinic of Om El Masryeen General hospital, from March 2019 to September 2019. One hundred pregnant women were divided equally into two groups: Group I (study group) included women suffering from recurrent unexplained first trimester pregnancy loss, and Group II (control group) included women with no history of pregnancy loss and at least one live birth at term. Age ranged from 18-40 years, and gestational age ranged from 6-13 weeks. Results: There was a significant increase in mean values of resistance index (RI) and pulsatility index (PI) of both uterine arteries in the study group compared to the control group. In addition, there was a significant increase in mean values of both RI and PI of uterine spiral arteries in the study group compared the control group. Conclusion: Transvaginal Doppler studies detected increased uterine blood flow resistance in recurrent unexplained first trimester pregnancy loss compared to normal population. Therefore, increased uterine blood flow impedance may serve as a cause for recurrent pregnancy loss.