Brace and Wolf (1989) reviewed 12 studies done through the 1960s in which amnionic fluid volume was assessed using these measurement techniques. Although fluid volume increased across gestation, they found that the mean value did not change significantly between 22 and 39 weeks-it was approximately 750 mL. There was considerable variation at each week of gestation, especially in the mid-third trimester, when the 5th percentile was 300 mL and the 95th percentile nearly 2000 mL. In contrast, Magann and colleagues (1997), using dye-dilution measurements, found that amnionic fluid volume rose with advancing gestation. Specifically, the average fluid volume was approximately 400 mL between 22 and 30 weeks, doubling thereafter to a mean of 800 mL. he volume remained at this level until 40 weeks and then declined by approximately 8 percent per week. The two reports difered in the regression methodology employed, and despite diferent conclusions, both identiied a wide normal range, particularly in the third trimester. This normal variation is similarly observed sonographically.
Amnionic fluid volume evaluation is a component of every standard sonogram performed in the second or third trimester (Chap. 10, p. 188). It may be measured using either of two semi-quantitative techniques, the single deepest pocket of fluid or the amnionic fluid index (API), which was described by Phelan and associates (1987). Both measurements are reproducible and, in the setting of a fluid abnormality, can be followed serially over time to assess trends and to aid communication among providers. For these reasons, semiquantitative assessment of amnionic luid is preferred to qualitative or subjective estimation (American College of Obstetricians and Gynecologists, 2016). Using either technique, a fluid pocket must be at least 1 em in width to be considered adequate. Fetal parts or loops of umbilical cord may be visible in the pocket, but they are not included in the measurement. Color Doppler is generally used to veriY that umbilical cord is not within the measurement.
his is also called the largest or maximal vertical pocket of amnionic fluid. he ultrasound transducer is held perpendicular to the floor and parallel to the long axis of the woman. Then, while scanning in the sagittal plane, the largest vertical pocket of luid is identified and measured. The single deepest pocket measurement is considered normal f above 2 cm and less than 8 cm, with values below and above this range indicating oligohydramnios and hydramnios, respectivey. hese thresholds are based on data from Chamberlain and associates (1984) and correspond to the 3rd and 97th percentiles. When evaluating twin pregnancies and other multi fetal gestations, a single deepest pocket of amnionic fluid is assessed in each gestational sac, again using a normal range of more than 2 em to less than 8 em (Hernandez, 2012; Society for Maternal-Fetal Medicine, 2013). The fetal biophysical profile similarly uses a single deepest vertical pocket threshold of more than 2 em to indicate normal amnionic luid volume. This is discussed further in Chapter 17 (p. 337).
28 Mild hydramnios 26 24E 22>;'20 ) 180.� 16Q:J=14= > 12·c0 10·cE 8« 6 4 2 0
FIGURE 11-1 Amnionic fluid index (AFI) according to gestational-age-specific and threshold values. The blue curves represent the 25th, 50th, and 975th AFI percentile values, based on the nomogram by Moore (1o990). Red and tan curves represent 50th percentile values for AFI from Machado (2007) and from Hinh and Ladinsky (2005), respectively. The light blue and yellow shaded bars indicate threshold values used to define hydramnios and oligohydramnios, respectively.