In addition, Moise 16 found the MVP to be superior to the AFI in diagnosing oligohydramnios using a MVP less than 2 cm. They concluded the MVP was a better choice to make the diagnosis of oligohydramnios, as it leads to fewer diagnoses of oligohydramnios and a decrease in inductions. Nabhan and Abdelmoula 15 performed a meta-analysis comparing the AFI and MVP and found that neither was superior in the prevention of poor perinatal outcomes however, the AFI resulted in a higher frequency of oligohydramnios being diagnosed (RR, 2.39 95% CI, 1.73 to 3.28) as well as an increase in inductions (RR, 1.92 95% CI, 1.5 to 2.46) and cesarean delivery for fetal distress (RR, 1.46 95% CI, 1.08 to 1.96). 14 More recently, others have found the MVP superior to the AFI. Attempts to determine which ultrasound measurement technique was superior at predicting poor pregnancy outcomes found that the AFI was superior.

Studies that have compared estimates of AFV by ultrasound (MVP and AFI) with actual measurements taken by the dye-dilution technique demonstrate that MVP and AFI measurements are poor predictors of actual AFV. 28.1) demonstrates very similar appearing curves. 28.4) with the actual measured volume (see Fig. Comparing the ultrasound estimation of the AFV by the AFI ( Fig. Finally, the investigators reported the interobserver and intraobserver variation to be 3.1% and 6.7%, respectively, which is acceptable for this commonly performed procedure. The variation in the AFI at the 5th percentile was less than that of the 95th percentile, but it still varied by as much as 2.5 cm. At the 95th percentile, the AFI at 35 to 36 weeks of gestation was 24.9 cm it was 19.4 cm at 41 weeks of gestation. 13 Moore and Cayle 13 performed a cross-sectional study of 791 normal pregnancies the 5th and 95th percentile of the AFI varied for each gestational age. The sum of the MVP in each quadrant equals the AFI. The deepest pocket of AF is measured in each quadrant, making sure that the ultrasound transducer is perpendicular to the floor and that fetal body parts and the umbilical cord do not interfere with the vertical measurement ( Fig. After 20 weeks, the uterus is divided into four equal quadrants, as shown in Fig. Subsequently, many authors 11-13 proposed a four-quadrant assessment of AF, referred to as the amniotic fluid index (AFI). These lower values of the MVP (1 cm or less) identified fetuses at risk, but the low sensitivity for identifying the majority of pregnancy complications associated with oligohydramnios was unacceptable and prompted other investigators to select higher cutoff values. 8 Chamberlain and colleagues 9 and Mercer and colleagues 10 found that perinatal morbidity and mortality rates were increased with an MVP of less than 1 and 0.5 cm, respectively. The diagnosis of oligohydramnios is a consideration when the fundal height is small for gestational age (SGA) or the fetus is easily palpated.Įarly ultrasound estimations of AFV were made by measuring the maximum vertical pocket (MVP) of AF. Polyhydramnios may be present if the maternal uterus is large for gestational age (LGA) or if the fetus cannot be easily palpated or is ballotable. However, AF disorders should be suspected when the uterus measures too large or too small for gestational age. Ultrasound has largely replaced clinical assessment of AFV based on the Leopold maneuver or fundal height measurements. Landon MD, in Gabbe's Obstetrics: Normal and Problem Pregnancies, 2021 Ultrasound Assessment of Amniotic Fluid Volume Typical limits for amniotic fluid assessment by AFI are defined in Table 24.1.

The absolute maximum normal AFI is less than 24 or 25 cm. Above the 95th percentile is approximately 20 cm and below the 5th percentile is approximately 7 cm. The mean AFI is approximately 12-14 cm throughout most of pregnancy, declining after 33 weeks. also established 5th-95th percentiles, respectively, as 4.2-14.9 cm for AFI at gestational age 37-41 weeks. Moore and Cayle established the mean and outer boundaries (5th and 95th percentiles, respectively) for the AFI from 16-42 weeks’ gestation in a study of 791 normal pregnancies. The AFI correlates closely with actual amniotic fluid volume as determined by dye-dilution techniques for normal fluid volume, although it loses its predictive value at the upper and lower ends of the extreme.

82 It uses the maternal umbilicus as the central reference point and requires the transducer to be oriented in the longitudinal axis of the maternal abdomen and be kept perpendicular to the floor during scanning.

Martin MBBS, FRACP, in Fanaroff and Martin's Neonatal-Perinatal Medicine, 2020 Amniotic Fluid Index MeasurementĪmniotic fluid index is the summation of the vertical diameter of the largest pocket in each of the four quadrants, as proposed by Phelan and Rutherford.

Amniotic fluid index