The fetus swallows amniotic fluid, removing water and electrolytes, and replaces them through urination with metabolic byproducts such as urea, creatinine, and uric acid. In the later stages of pregnancy, fetal swallowing and urination play a major role in the volume and composition of the amniotic fluid. Because of fetal respiration in utero, the fetal pulmonary surfactants produced by alveolar cells of the fetal lungs mix with and can be evaluated using amniotic fluid. Early in gestation (before keratinization of the skin), a transudate passes through the skin of the fetus and makes a small contribution to the amniotic fluid volume. Water and solutes are exchanged between the fetus and its surrounding medium through several mechanisms: (1) intestinal absorption after fetal swallowing of amniotic fluid (2) capillary exchange in the pulmonary system as the alveoli of the fetal lungs are bathed with amniotic fluid and (3) fetal urination. However, as gestation progresses, the fetus plays more of an active role in the composition of the fluid. Initially, amniotic fluid is produced by the amnion and the placenta, and its composition is similar to that of a dialysate of plasma. The dynamics of amniotic fluid formation and its composition change throughout fetal gestation. 14.1 Schematic diagram of a fetus in utero. Although studies have investigated many substances as potential biochemical markers of disease, few substances (e.g., phospholipids) have demonstrated reliable clinical utility and value.įig. Fetal cells and many biochemical compounds, such as electrolytes, nitrogenous compounds, proteins, enzymes, lipids, and hormones, are present in the amniotic fluid. Amniotic fluid protects the fetus while enabling fetal movement and plays an important role in numerous biochemical processes. The amnion, a membrane composed of a single layer of cuboidal epithelial cells, surrounds the fetus and is filled with this fluid. The specialized laboratory techniques required to detect numerous and varied genetic and metabolic disorders using amniotic fluid are beyond the scope and intent of this text and therefore are not discussed.Īmniotic fluid is the liquid medium that bathes a fetus throughout its gestation ( Fig. The study of amniotic fluid is performed primarily for three reasons: (1) to enable antenatal diagnosis of genetic and congenital disorders early in fetal gestation (15–18 weeks), (2) to assess fetal pulmonary maturity later in the pregnancy (32–42 weeks), and (3) to estimate and monitor the degree of fetal anemia caused by isoimmunization or infection.īy far the most frequently performed tests on amniotic fluid in the clinical laboratory are used to assess fetal lung maturity (FLM) and fetal anemia resulting from an isoimmune disease, which are discussed in this chapter. Advancements in technology have provided new technical methods and clinical applications for amniotic fluid analysis. With the use of ultrasound, amniocentesis is now a common and relatively safe obstetric procedure. polyhydramnios (also called hydramnios).
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