24 - Amniotic Fluid Volume

Abstract

Amniotic fluid surrounds and protects the developing fetus during pregnancy, and its volume is from surface exchange in early pregnancy and the fetal renal system in later pregnancy. The assessment of amniotic fluid volume (AFV) can be done precisely by dye-dilution techniques or measured at the time of cesarean delivery. The dye-dilution techniques require an amniocentesis, are invasive, time consuming, and require laboratory support, and the measurement at cesarean can only be done and delivery, making that technique impractical for the serial assessment of AFV. Because of these limitations AFV is estimated by ultrasound. The current techniques to estimate AFV include the subjective assessment, and the objective assessments using the amniotic fluid index (AFI) and the single deepest pocket technique (SDP). These ultrasound estimates of the AFV correlate well with normal (dye-determined or directly measured) volumes but poorly with true oligohydramnios and polyhydramnios. The use of color Doppler in estimating AFV does not help in identifying true oligohydramnios but instead labels more normal fluid volumes as oligohydramniotic. The use of the statistical method, quantile regression, is the best method to identify what is a normal AFV across gestation. In twin pregnancies, the amniotic fluid volume is best estimated using the SDP technique. The SDP technique compared to the AFI is the superior technique to use in antenatal testing to evaluate AFV in singletons. Polyhydramnios is seen with fetal anomalies, maternal diabetes, fetal macrosomia, immune and nonimmune fetal hydrops, or fetal infection, or may be idiopathic. The best ultrasound measurement to identify polyhydramnios is uncertain.