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It is generally agreed that amniotic fluid represents in great part a transudation from the maternal vessels, but many authorities consider that a portion of it is derived from urinary secretion of the foetus.
—J. Whitridge Williams (1903)
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At the time Williams wrote this, the fetal kidney was thought by many to be nonfunctional. Since that time, however, much has been learned of this complex multifunctional liquor amnii. Amnionic fluid serves several roles during pregnancy. Fetal breathing of amnionic fluid is essential for normal lung growth, and fetal swallowing permits gastrointestinal (GI) tract development. Amnionic fluid also creates a physical space for fetal movement, which is necessary for neuromusculoskeletal maturation. It further guards against umbilical cord compression and protects the fetus from trauma. Amnionic fluid even has bacteriostatic properties. Abnormalities of volume may result from fetal or placental pathology—indicating a problem with fluid production or its circulation. These volume extremes may be associated with increased risks for adverse pregnancy outcome.
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NORMAL AMNIONIC FLUID VOLUME
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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). The 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.
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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 transcutaneous flow—across fetal skin. Fetal urine production begins between 8 and 11 weeks’ gestation, but it does not become a major component of amnionic fluid until the second trimester, which 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 neonates can experience significant fluid loss across their skin.
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With advancing gestation, four pathways play a major role in amnionic fluid volume regulation (Table 11-1). First, fetal urination is the primary source of amnionic fluid in the second half of pregnancy. By term, fetal urine production may exceed 1 liter per day, and the entire amnionic fluid volume is recirculated on a daily basis. Fetal urine osmolality is similar to that of amnionic fluid and significantly hypotonic to that of maternal and fetal plasma. Specifically, the osmolality of maternal and fetal plasma approximates 280 mOsm/mL, whereas that of amnionic fluid is about 260 mOsm/L. ...