The ultrasonic estimation of amniotic fluid volume (AFV) has become an integral component of pregnancy assessment. In early gestations, targeted sonography facilitates a survey of fetal anatomy. Excessive or deficient AFV can reflect underlying problems. Low fluid volumes have been associated with renal dysplastic processes, genitourinary obstructions, and severe growth restriction. High volumes have been identified with neural tube defects, central nervous system abnormalities affecting fetal swallowing, gastrointestinal obstructions, infections, and immune or nonimmune hydrops.

Later in pregnancy, the estimation of AFV is an integral component of antenatal testing for the assessment of fetal health. Abnormalities of AFV encountered at these times have been associated with poor perinatal outcome. In the United States, the most frequently used method of fetal health assessment is probably the combination of the nonstress test (NST) and an amniotic fluid index (AFI). These tests are usually performed weekly on most at-risk patients, except those with pregnancies complicated by insulin-dependent diabetes mellitus, intrauterine growth restriction, and postdatism, in which the testing is performed twice a week. If a NST is nonreactive or if a contraction stress test (CST) is equivocal, often the next step is to evaluate fetal well-being with a biophysical profile that incorporates AFV assessment as one of its five components.

Aberrations of AFV are thought to reflect perturbations of the uterine environment. If chronic uteroplacental insufficiency occurs, the fetus is expected to shunt blood to its brain, heart, and adrenal glands at the expense of the rest of its body. Decreased renal blood flow, perfusion, and urinary output lead to decreased AFV because fetal urine is the major contributor to amniotic fluid after midgestation.

Ultrasonic estimates of AFV can have a great impact on the subsequent obstetric management of patients who undergo antenatal testing. In a woman in whom the ultrasonic estimate of AFV is low, the diagnosis of oligohydramnios frequently leads to additional interventions even if other components of antenatal testing are reassuring of fetal health. These interventions can include additional antenatal testing, hospitalization, oral or intravenous hydration or both, and possibly cervical ripening/labor induction with a high risk of abdominal delivery, particularly for failed inductions in the parturient with an unfavorable cervix. Because a low ultrasonic estimate of AFV can lead to delivery of a preterm infant with potentially mortal consequences, there is a need to investigate the evidence in the obstetric literature regarding the association between the ultrasonic estimate of AFV and actual AFV and the relationship between abnormal AFV and perinatal outcome.