Relation of mild idiopathic polyhydramnios to perinatal outcome.

The relation between clinically obvious polyhydramnios and poor perinatal outcome has been described. Much less is known about mild, unexplained polyhydramnios, which usually is initially suggested by sonographic examination late in gestation. The purpose of the present investigation was to relate mild idiopathic polyhydramnios to perinatal outcome. Mild polyhydramnios was defined sonographically as an amniotic fluid index of 24.1-39.9 during fetal biophysical testing. All subjects had singleton gestations not complicated by conditions known to predispose to polyhydramnios. We diagnosed mild polyhydramnios in 97 of 1177 patients (8.2%) undergoing fetal testing at 26-42 gestational weeks. Premature delivery, intrapartum complications, and neonatal depression were no more frequent in the pregnancies complicated by mild, unexplained polyhydramnios than in a comparable control group with normal fluid volume. The mild polyhydramnios group showed a significantly higher incidence of birth weight greater than 4000 g than did the control group (18.6 versus 8.6%; P less than .05). We conclude that mild idiopathic polyhydramnios in late gestation is relatively common. Except for a higher incidence of large for gestational age fetuses, this condition by itself is not associated with an increased risk of adverse perinatal outcomes.