Oligohydramnios, defined either as an amniotic fluid index (AFI) of less than 5, an AFI less than the fifth percentile for gestational age, or a maximum vertical pocket of less than 2 cm is a complication of pregnancy that leads to concern for both the patient and their provider. After approximately 16 weeks’ gestation, amniotic fluid concentration is determined by fetal urine excretion, secretion by the fetal respiratory tract, and fetal swallowing. A defect in any of these 3 can lead to either oligo- or polyhydramnios. The most common cause of oligohydramnios is ruptured membranes. Other causes include congenital anomalies and postdates pregnancies. However, when the etiology is unclear, management of this complication can be confusing at best. Does early delivery prevent impending fetal or perinatal complications, or does it lead to an increase in neonatal complications?

Data to help sort out these questions are muddled by confounders that are potential comorbidities leading to increased complication rates. Manning et al assessed oligohydramnios in 120 patients referred for ultrasounds secondary to intrauterine growth restriction (IUGR). As expected, they found a 10-fold increase in perinatal morbidity in women with decreased amniotic fluid volume. Chamberlain et al reviewed more than 7000 high-risk patients referred for ultrasound in an effort to associate perinatal outcomes with amniotic fluid volume. Not surprisingly, they found that decreased amniotic fluid was associated with markedly increased perinatal morbidity. However, their population included patients with congenital anomalies and IUGR. The ideal population from which to answer the question of potential complications from oligohydramnios would be a group of pregnant women with no other known pregnancy, obstetric, or medical complications.

Zhang et al attempted to find a population of women with isolated oligohydramnios to answer this question. Using data from the Routine Antenatal Diagnostic Imaging with Ultrasound trial in which women underwent routine ultrasound screening, they identified a population of 113 women with isolated oligohydramnios. There was no association with adverse perinatal outcomes in this group of women.

Whereas the data to support early delivery of women with oligohydramnios to improve perinatal morbidity are weak at best, the data that show increased morbidity of late preterm delivery are clear. Late preterm infants are at increased risk for significant respiratory morbidity as well as increased risk for complications of prematurity (intraventricular hemorrhage, necrotizing enterocolitis, hyperbilirubinemia). Early data showing potential long-term consequences of late preterm birth, including an increased risk of cerebral palsy and mental retardation, make delivery of this vulnerable group unwarranted unless the risks of prolonged pregnancy outweigh the benefits of early delivery. Currently there are no data that support early delivery of women with oligohydramnios.