ObjectivesTo compare maternal and perinatal outcomes in pregnancies with transient and persistent idiopathic polyhydramnios to those with normal amniotic fluid volume.MethodsThis retrospective cohort study included subjects delivering a singleton pregnancy between January 1, 2015, and June 30, 2020, with sonography ≥26 weeks' gestation demonstrating transient or persistent idiopathic polyhydramnios (amniotic fluid index [AFI] ≥24.0 cm) or normal AFI (referent group). The primary maternal outcome was a composite of spontaneous preterm delivery <37 weeks, cesarean delivery for malpresentation, abnormal labor progress, or nonreassuring fetal status, operative vaginal delivery, hemorrhage requiring transfusion, and umbilical cord prolapse. The primary perinatal outcome was a composite of birthweight >4500 g, fetal or neonatal death, 5‐minute Apgar score <7, and neonatal intensive care unit admission for >24 hours.ResultsPatients with transient polyhydramnios (n = 259) exhibited maternal outcomes similar to those of the referent group (n = 435) but had significantly increased odds for the primary perinatal outcome (odds ratio [OR] 1.70, 95% confidence interval [CI] 1.15–2.53; P = .008) and for birthweight ≥4500 g (OR 8.70, 95% CI 1.89–40.0; P = .005). Persistent polyhydramnios (n = 176) was significantly associated with both the primary maternal (OR 1.93, 95% CI 1.27–2.95, P = .002) and primary perinatal outcome (OR 2.15, 95% CI 1.40–3.30; P < .001), and individually with cesarean delivery for abnormal labor (OR 3.22, 95% CI 1.55–6.68; P = .002) and birthweight ≥4500 g (OR 8.97, 95% CI 1.84–43.6; P = .007).ConclusionsTransient idiopathic polyhydramnios does not impact maternal outcomes but is associated with increased odds of newborn birthweight >4500 g. Persistent polyhydramnios is associated with both adverse maternal and perinatal outcomes.