Sildenafil citrate is a vasodilator used in erectile dysfunction and pulmonary hypertension. We tested whether it reduces emergency operative births for fetal compromise and improves fetal or uteroplacental perfusion in labor in a phase 2 double-blind randomized controlled trial. Women at term in early labor or undergoing scheduled induction of labor at Mater Mother's Hospital, Brisbane, Australia, were randomly allocated 50 mg of sildenafil citrate orally 8hourly up to 150 mg or placebo. Intrapartum fetal monitoring followedRoyal Australian and New Zealand College of Obstetricians andGynaecologists guidelines. Primary outcomes were (1) emergency operative birth (by cesarean delivery or instrumental vaginal birth)forintrapartum fetal compromise and (2) mean indices of fetalanduteroplacental perfusion using Doppler ultrasound. Analysiswas by intention-to-treat. ANZCTRN12615000319572 RESULTS: Between September 2015 and January 2019, 300 women were randomized equally to sildenafil citrate or placebo. Sildenafil citrate reduced the risk of emergency operative birth by 51% (18% vs 36.7%; relative risk, 0.49, 95% confidence interval, 0.33-0.73, P=.0004, number needed to treat= 5 [3-11]). There was no difference in indices of fetal and uteroplacental perfusion, but these were ascertained in only 71 women. Sildenafil citrate reduced the risk of meconium-stained liquor or pathologic fetal heart rate patterns by 43% (25.3% vs 44.7%; relative risk, 0.57, 95% confidence interval, 0.41-0.79, P=.0005), but its effects on fetal scalp sampling rates (2.0% vs 6.7%; relative risk, 0.30, 95% confidence interval, 0.08-1.07, P=.06) and adverse neonatal outcome (20.7% vs 21.3%; relative risk, 0.97, 95% confidence interval, 0.62-1.50, P=.89) were inconclusive. Only 3.6% of maternal levels of sildenafil citrate or its metabolite were detected in cord blood. No differences in maternal adverse events were seen. Sildenafil citrate reduced operative birth for intrapartum fetal compromise, but much larger phase 3 trials of its effects on mother and child are needed before it can be routinely recommended.