Amnionic fluid serves several roles during pregnancy. It creates a physical space for fetal movement, which is necessary for normal musculoskeletal development. It permits fetal swallowing—essential for gastrointestinal tract development, and fetal breathing—necessary for lung development. Amnionic fluid guards against umbilical cord compression and protects the fetus from trauma. It even has bacteriostatic properties. Amnionic fluid volume abnormalities may reflect a problem with fluid production or its circulation, such as underlying fetal or placental pathology. These volume extremes may be associated with increased risks for adverse pregnancy outcome.
Normal Amnionic Fluid Volume
Amnionic fluid volume increases from approximately 30 mL at 10 weeks to 200 mL by 16 weeks and reaches 800 mL by the mid-third trimester (Brace, 1989; Magann, 1997). This fluid is approximately 98-percent water. A full-term fetus contains roughly 2800 mL of water, and the placenta another 400 mL, such that the term uterus holds nearly 4 liters of water (Modena, 2004). Abnormally decreased fluid volume is termed oligohydramnios, whereas abnormally increased fluid volume is termed hydramnios or polyhydramnios.
Early in pregnancy, the amnionic cavity is filled with fluid that is similar in composition to extracellular fluid. During the first half of pregnancy, transfer of water and other small molecules takes place across the amnion—transmembranous flow, across the fetal vessels on placental surface—intramembranous flow, and across fetal skin. Fetal urine production begins between 8 and 11 weeks, but it does not become a major component of amnionic fluid until the second trimester. This latter observation explains why fetuses with lethal renal abnormalities may not manifest severe oligohydramnios until after 18 weeks. Water transport across the fetal skin continues until keratinization occurs at 22 to 25 weeks. This explains why extremely preterm infants can experience significant fluid loss across their skin.
With advancing gestation, four pathways play a major role in amnionic fluid volume regulation (Table 11-1). First, fetal urination is the primary amnionic fluid source by the second half of pregnancy. By term, fetal urine production may exceed 1 liter per day—such that the entire amnionic fluid volume is recirculated on a daily basis. Fetal urine osmolality is significantly hypotonic to that of maternal and fetal plasma and similar to that of amnionic fluid. Specifically, the osmolality of maternal and fetal plasma is approximately 280 mOsm/mL, whereas that of amnionic fluid is about 260 mOsm/L. This hypotonicity of fetal urine—and thus of amnionic fluid—accounts for significant intramembranous fluid transfer across and into fetal vessels on the placental surface, and thus into the fetus. This transfer reaches 400 mL per day and is a second regulator of fluid volume (Mann, 1996). In the setting of maternal dehydration, the resultant increase in maternal osmolality favors fluid transfer from the fetus to the mother, and then from the amnionic fluid ...