The role of amniotic fluid assessment in indicated preterm delivery

Measuring amniotic fluid pockets with ultrasound is an efficient and reasonably reliable method of evaluating amniotic fluid volume and categorizing relative risk of perinatal morbidity. The most commonly used ultrasound criteria for oligohydramnios, SDP <2 cm and AFI <5 cm, assign approximately 2%-3% and 4%-5% of late preterm pregnancies into the "low amniotic fluid" category. The AFI offers somewhat greater sensitivity and greater precision but has less specificity for predicting perinatal morbidity than does the SDP. Thus, before 34 weeks, use of the AFI <5 cm as a criterion for intensive fetal monitoring, but not as sole criteria for delivery, is recommended. In pregnancies beyond 34 weeks, use of either AFI or SDP to diagnose oligohydramnios can be expected to reliably identify fetuses at risk for compromised perinatal outcome especially if replicate measurements are confirmatory. In such cases, care must be taken to identify comorbid conditions that, together with oligohydramnios, may place the fetus at significant risk. In such cases, delivery is the recommended intervention.