The fluid filling the amniotic sac serves several important functions. It provides a medium in which the fetus can readily move, cushions the fetus against possible injury, helps maintain an even temperature, and provides useful information concerning the health and maturity of the fetus. The origin of amniotic fluid is not completely understood. In early pregnancy, it is produced by the amniotic membrane covering the placenta and the cord. As the pregnancy progresses, it is believed to be primarily a byproduct of fetal pulmonary secretions, urine, and metabolic products from the intestinal tract.
Initially, amniotic fluid is produced from the amniotic membrane cells. Later, most of it is derived from the maternal blood. The volume increases from about 30 mL at 2 weeks gestation to 350 mL at 20 weeks. After 20 weeks, the volume ranges from 500 to 1000 mL. The volume of amniotic fluid changes continuously because of fluid movement in both directions through the placental membrane. Later in pregnancy, the fetus contributes to AFVs through excretion of urine and swallowing of amniotic fluid. The fetus also absorbs up to 400 mL of amniotic fluid every 24 hours through its gastrointestinal tract, bloodstream, and umbilical artery exchanges across the placenta. Probably, some fluid is also absorbed by direct contact with the fetal surface of the placenta. Amniotic fluid contains castoff cells from the fetus and resembles extracellular fluid with suspended, undissolved material. It is slightly alkaline and contains albumin, urea, uric acid, creatinine, lecithin, sphingomyelin, bilirubin, fat, fructose, epithelial cells, leukocytic enzymes, and lanugo hair.
When amniocentesis is advised early in pregnancy (15–18 weeks), the purpose is to study the fetal genetic makeup and to determine developmental abnormalities. Fetal cells are separated from the amniotic fluid by centrifugation and are placed in a tissue culture medium so that they can be grown and harvested for subsequent karyotyping to identify chromosome disorders. Testing in the third trimester is done to determine fetal age and well-being, to study blood groups, or to detect amnionitis.
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