Objectives: To compare the effect of vaginal Sildenafil Citrate (SC) versus the Placebo on the pregnancy outcome in women with unexplained recurrent spontaneous abortion (URSA) and to estimate the impact of SC on the uterine arteries blood flow indices. Methods: A double-blind, randomized, and placebo-controlled clinical trial was performed on 130 patients with URSA. They were treated with vaginal SC (n=65) and vaginal Placebo (n=65) starting from day 21-23 of the cycle until menstruation and continued up to 12 weeks gestation if pregnancy occurred. The uterine artery pulsatility index (PI) and resistance index (RI) were measured at randomization and at 12 weeks gestation. The primary outcome measure was the live birth. Secondary outcome measures were the differences between both groups regarding PI and RI, the take-home babies, the drug-induced side-effects, and the pregnancy-related complications. Results: There were significantly higher rates of live births (70.8% vs. 50.8%; p=0.0237) and take-home babies (61.5% vs. 36.9%; p=0.007), significantly lower number of pregnancies complicated by intrauterine growth restriction (4 vs. 12; p=0.046) and pregnancy-induced hypertension (4 vs. 13; p=0.03) and no significant difference regarding treatment-related adverse-effects in the sildenafil group compared to placebo group. The means of uterine artery PI and RI measured at 12 weeks of gestation were significantly lower in the Sildenafil group than the Placebo group (p =0.0013 &0.007; respectively), however, this difference was not significant at cycle day 21-23 before treatment (p >0.05). Conclusions: Sildenafil citrate could be potentially effective to increase uterine perfusion in early pregnancy, decrease the incidence of abortion and increase live birth and take-home baby rates in patients with the history of early unexplained recurrent spontaneous abortion.