Amnionic fluid serves several roles during pregnancy. It creates a physical space for the fetal skeleton to shape normally, promotes normal fetal lung development, and helps to avert compression of the umbilical cord. The most common intrinsic abnormalities that are encountered clinically are either too much or too little amnionic fluid.
Normally, amnionic fluid volume reaches 1 L by 36 weeks and decreases thereafter to less than 200 mL at 42 weeks (Table 21-1). Diminished fluid is termed oligohydramnios. Somewhat arbitrarily, more than 2 L of amnionic fluid is considered excessive and is termed hydramnios or polyhydramnios. In rare instances, the uterus may contain an enormous quantity of fluid, with reports of as much as 15 L. In most instances, chronic hydramnios develops, which is the gradual increase of excessive fluid. In acute hydramnios, the uterus may become markedly distended within a few days.
Table 21-1. Typical Amnionic Fluid Volume |Favorite Table|Download (.pdf)
Table 21-1. Typical Amnionic Fluid Volume
Amnionic Fluid (mL)
Measurement of Amnionic Fluid
Over the past decades, a number of sonographic methods have been used to measure the amount of amnionic fluid. Phelan and colleagues (1987) described quantification using the amnionic fluid index—AFI. This is calculated by adding the vertical depths of the largest pocket in each of four equal uterine quadrants. According to their calculations, significant hydramnios is defined by an index greater than 24 cm. Magann and colleagues (2000) performed a cross-sectional study of longitudinal changes in the amnionic fluid index in normal pregnancies (Fig. 21-1). Their curve and the ones of Hinh and Ladinsky (2005) and Machado and colleagues (2007) show a peak AFI at approximately 32 weeks followed by a steady decline until 42 weeks. Normal values for multifetal pregnancy have been provided by Porter and associates (1996) and Hill and co-workers (2000) and are discussed further in Chapter 39 (Antepartum Surveillance).
Normal percentiles for amnionic fluid index (AFI) during uncomplicated pregnancy. Black curves represent the 5th, 50th, and 95th percentile values from Magann and co-workers (2000). Red and tan curves represent 50th percentile values from Machado and colleagues (2007) and from Hinh and Ladinsky (2005), respectively.
The group from the University of Mississippi has performed several investigations to assess the sonographic accuracy of AFI evaluation. Magann and associates (1992) compared AFI values with measurements obtained by dye dilution. They used this technique to measure ...