Amniotic Fluid
Amniotic fluid is a body fluid that is unique to pregnancy. The volume of amniotic fluid increases progressively through gestation until approx 34 wk, when the volume begins to decrease. It is approx 200 to 300 mL at 16 wk, 400 to 1400 mL at 26 wk, 300 to 2000 mL at 34 wk, and 300 to 1400 mL at 40 wk (35). The volume of amniotic fluid at term is approx 800 mL and constitutes only 6% of the total maternal weight gain by fluids (27) (Table 7). During gestation, a woman with a single fetus will gain, on average, 12.5 kg (27.5 lb), of which 4.85 kg (10.7 lb) constitute the fetus, placenta, and amniotic fluid (27). As a percent of the total weight gained by the mother, the products of conception rise from about 10% at 10 wk to 20% at 20 wk, 30% at 30 wk, and 40% at term (Table 7).
In the first trimester, the origin of amniotic fluid is uncertain. Beginning in the second trimester, the origin and dynamics of amniotic fluid are better understood. Amniotic fluid volume is maintained by a balance of fetal fluid production (lung liquid and urine) and fluid reabsorption (fetal swallowing and flow across the amniotic membranes). Alterations in the volume of amniotic fluid are associated with a variety of pathological conditions. Oligohydramnios, or abnormally low amniotic fluid volume, is associated with intrauterine growth restriction and abnormalities of the fetal urinary tract. Polyhy-dramnios, or increased amniotic fluid volume, is associated with maternal diabetes mellitus, severe Rh isoimmunization disease, fetal esophageal atresia, multifetal pregnancy, enencephaly, and spina bifida (36).
During gestation, concentrations of glucose, urea nitrogen, and creatinine increase, whereas concentrations of sodium and osmolality decrease (19,37). These changes are not unexpected as, late in gestation, the primary source of amniotic fluid is fetal urine (see the appendix). The urine is normally hypotonic, and this low osmolality accounts for the relative hypotonicity of amniotic fluid (compared to maternal and fetal plasma). Early in pregnancy, there is little particulate matter in the amniotic fluid. As gestation progresses, fetal cells and hair are shed into the amniotic fluid, which can increase the turbidity of the fluid. As the fetal lung matures, production of phospholipid-containing lamellar bodies also increases the fluid turbidity. Finally, at term, amniotic fluid contains gross particles of vernix caseosa, the oily substance composed of sebum and desquamated epithelial cells covering the fetal skin (36).
Because amniotic fluid is derived from substances of fetal origin, it is often sampled and tested in order to monitor fetal development or detect fetal disease. Amniotic fluid is sampled routinely to obtain fetal cells for chromosomal analysis and detection of Down syndrome, trisomy 18, and a variety of other chromosomal abnormalities and to measure a-fetoprotein and acetylcholinesterase to detect neural tube defects (see Chapters 5 and 6 for more detail). It is also sampled later in pregnancy for the analysis of fetal lung-
Table 7
Average Weight of Products of Conception, Plasma, Red Cells, Extracellular Fluid, and Maternal Body by Gestational Age (in Grams)
Weeks of pregnancy
Table 7
Average Weight of Products of Conception, Plasma, Red Cells, Extracellular Fluid, and Maternal Body by Gestational Age (in Grams)
Weeks of pregnancy
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