Normally, amnionic fluid volume increases to about 1 L by 36 weeks and decreases thereafter to only 100 to 200 mL or less postterm. In rare instances, the volume of amnionic fluid may fall far below the normal limits and occasionally be reduced to only a few milliliters. Diminished fluid volume is termed oligohydramnios and is sonographically defined as an amniotic fluid index (AFI) of 5 cm or less (see Chapter 9). In general, oligohydramnios developing early in pregnancy is less common and frequently has a bad prognosis. By contrast, diminished fluid volume may be found often with pregnancies that continue beyond term. The risk of cord compression, and in turn fetal distress, is increased with diminished amnionic fluid in all labors, but especially in postterm pregnancy.
MEASUREMENT OF AMNIONIC FLUID
The amnionic fluid index (AFI) is calculated by dividing the pregnant uterus into four quadrants and placing the transducer on the maternal abdomen along the longitudinal axis. The vertical diameter of the largest amnionic fluid pocket in each quadrant is measured with the transducer head held perpendicular to the floor. The measurements are summed and recorded as the AFI. Normal AFI values for normal pregnancies from 16 to 42 weeks are listed in Appendix B, “Ultrasound Reference Tables.” The amnionic fluid index is reasonably reliable in determining normal or increased amnionic fluid but is less accurate in determining oligohydramnios. Several factors may modulate the amnionic fluid index, including altitude, and maternal fluid restriction or dehydration.
Several conditions have been associated with diminished amnionic fluid (Table 10-1). Oligohydramnios is almost always evident when there is either obstruction of the fetal urinary tract or renal agenesis. Anywhere from 15 to 25 percent of cases are associated with the fetal anomalies shown in Table 10-2. A chronic leak from a defect in the membranes may reduce the volume of fluid appreciably, but most often labor soon ensues. Exposure to angiotensin-converting enzyme inhibitors has also been associated with oligohydramnios (see Chapter 8).
TABLE 10-1. Conditions Associated with Oligohydramnios
TABLE 10-2. Congenital Anomalies Associated with Oligohydramnios
Fetal outcome is poor with early-onset oligohydramnios and only half survive. Preterm delivery and neonatal death are also common. Oligohydramnios is associated with adhesions between the amnion and fetal parts and may cause serious deformities including amputation. Moreover, in the absence of amnionic fluid, the fetus is subjected to pressure from all sides and musculoskeletal deformities such as clubfoot are observed frequently.
Pulmonary hypoplasia is associated with early-onset oligohydramnios and occurs in about 15 percent of fetuses with oligohydramnios identified during the first two trimesters. There are several possibilities that may account for pulmonary hypoplasia seen in these pregnancies. First, thoracic compression may prevent chest wall excursion and lung expansion. Second, lack of fetal breathing movements decreases fluid inflow to the lung. The third and most widely accepted model suggests that there is a failure to retain amnionic fluid or increased outflow with impaired lung growth and development. Thus, the appreciable volume of amnionic fluid inhaled by the normal fetus plays an important role in growth of the lung.
OLIGOHYDRAMNIOS IN LATE PREGNANCY
An amnionic fluid index of less than 5 cm after 34 weeks is associated with an increased risk of adverse perinatal outcomes (Table 10-3). For example, a pregnancy with an intrapartum amnionic fluid index of less than 5 cm is at an increased risk for variable fetal heart rate decelerations, cesarean delivery for fetal distress, and 5-minute Apgar score of less than 7.
TABLE 10-3. Pregnancy Outcomes (in percent) in 147 Women with Oligohydramnios at 34 Weeks
Infusion of crystalloid to replace pathologically diminished amnionic fluid has most often been used during labor to prevent umbilical cord compression. The technique for amnioinfusion is described in Chapter 13.
For further reading in Williams Obstetrics, 23rd ed.,
see Chapter 21, “Disorders of Amnionic Fluid Volume.”