To compare maternal and perinatal outcomes in pregnancies with transient and persistent idiopathic polyhydramnios to those with normal amniotic fluid volume.This 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 4500 g, fetal or neonatal death, 5-minute Apgar score 24 hours.Patients 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 4500 g. Persistent polyhydramnios is associated with both adverse maternal and perinatal outcomes.