turnover, post‐translational modifications, subcellular localization, and protein–protein interactions. Initial efforts aimed at proteomic analysis of AF led to the identification of hundreds of proteins,80, 81 although this strategy has been of limited value in testing for risk of preterm birth,82–84 fetal aneuploidy,85 or Rh‐negative pregnancy.86
Lipids
Lipids do not seem to be transported across the placenta and are not found in AF after maternal injection. Biezenski87 described the lipid content of AF from the 26th week of gestation. The phospholipids measured included lysophosphatidylcholine, sphingomyelin, phosphatidylcholine, inositol, serine, ethanolamine, phosphatidic acid, and cardiolipin. Biezenski also established values for total fatty acids, including palmitic acid, palmitoleic acid, stearic acid, oleic acid, and linoleic acid. This author concluded that total lipid was about 1–2 percent of that found in maternal plasma during pregnancy and about 5 percent of that found in fetal plasma.
Phosphatidylserine normally found in AF and in the placenta is not present in maternal plasma, whereas the sphingomyelin content of AF is much lower than in plasma.87 Total cholesterol represents roughly one‐third of the total lipids in AF. Biezenski87 observed that the lipid profile remained essentially unchanged in the third trimester, despite the striking increase in AFV during this period. Near term, the placenta prevents the transfer of maternal esterified fatty acids in the form of phospholipids, triglycerides, or cholesteryl esters, although appreciable amounts of unesterified fatty acids and free cholesterol are transferred.88 AF collected more than 2 weeks after fetal death shows increased total lipid concentrations due mainly to increased free cholesterol, unesterified fatty acids, and hydrocarbons.
Pomerance et al.89 observed no specific diagnostic lipid pattern in their detailed lipid analyses of various complicated pregnancies, including hemolytic disease of the newborn, toxemia of pregnancy, diabetes, anencephaly, and hydramnios. Gardella et al.90 found an association between lipopolysaccharide‐binding protein and soluble CD14 and preterm labor.
In pregnancies affected by autosomal recessive Smith–Lemli–Opitz syndrome (SLOS), Dallaire et al.91 and Tint et al.92 found that low cholesterol and elevated 7‐dehydrocholesterol (7‐DHC) values were pathognomonic of the disorder. Mutation analysis of the 7‐dehydrocholesterol reductase gene93 on DNA derived from chorion villus samples or AF cells94, 95 has brought precision to this prenatal diagnosis. Observation of low maternal serum unconjugated estriol,96 or accumulation of 7‐ and 8‐DHC in AF,97 would prompt mutation analysis.98–100 Prenatal diagnosis can be made on the basis of malformations consistent with the syndrome, intrauterine growth restriction, and sterol analysis in AF or chorionic villi.101, 102
Other sterols in AF including lathosterol, desmosterol, lanosterol, and dimethylsterol, when deficient, may signal a prenatal diagnosis of lathosterolosis, desmosterolosis, X‐linked chondrodysplasia, and the Antley–Bixler syndrome.97
The fatty acid composition of AF103 differs considerably from that found in maternal plasma. Fetal renal excretion seems to be the origin of part of the free fatty acids in AF, at least during the third trimester. The immunosuppressive activity of AF may be due to lipid‐like factors providing a nonspecific immunoregulatory mechanism that prevents the immune rejection of the conceptus by the mother.104
Studies of bile acid concentrations in normal and pathologic pregnancy revealed elevated bile acid concentrations in the AF of fetuses with intestinal obstructions.105, 106 Such results are expected for all intestinal obstructions distal to the ampulla of Vater, where the fetal stomach content will be regurgitated into AF.107 In general, the mean bile acid concentrations in the AF were similar to those in the serum. However, in paired samples from individual patients, these two values did not correlate well.105
Gluck and Kulovich pioneered the analysis of AF phospholipids for the assessment of fetal pulmonary maturity.108 The surface‐active phospholipids lecithin (L) and sphingomyelin (S) originate from the fetal lungs. A marked increase in the production of lecithin occurs at about 35 weeks of gestation.109 As lecithin passes from the lung into the AF, an increase in the L/S ratio in AF occurs. The correlation of L/S ratio with gestational age is well established.110 Various pregnancy complications have a marked effect on the maturation of the fetal lung and hence the L/S ratio. Conditions that affect fetal lung maturation, including maternal hypertension, placental insufficiency, and diabetes mellitus, render the L/S ratio less valuable.108
Lamellar bodies store phospholipids that serve as pulmonary surfactant to reduce surface tension, which is essential for lung maturity. Lamellar body count111 and surfactant‐to‐albumin ratio in AF for predicting the risk of respiratory distress syndrome are equally accurate and to an important extent eliminate L/S ratio‐identified false‐positive cases of fetal lung maturity.112 Whereas the general consensus is that amniocentesis to determine fetal lung maturity should not guide timing of delivery,113 this remains controversial especially for rural obstetrics practices.114 At Mayo Clinic, lamellar body count (LBC) is the test of choice for fetal lung maturity, with reflex to L/S ratio when LBC is indeterminate (MJ Wick, personal communication).
Enzymes
Many enzymes have been found in the AF. Some have specific activities greater than those found in maternal serum, such as diamine oxidase115–117 and phosphohexose isomerase,116, 117 whereas others have greater activity in maternal serum, such as histaminase118 and creatine phosphokinase.119 The activity of some enzymes in fetal serum exceeds that found in AF (e.g. glucose‐6‐phosphate dehydrogenase, malate dehydrogenase, glutamic‐oxaloacetic transaminase, glutamic‐pyruvate transaminase, and leucine aminopeptidase).120, 121 Some enzymes were proposed as maturity indices: α‐galactosidase,122 pyruvate kinase,123 alkaline phosphatase, γ‐glutamyl transferase,124 and prolidase.125
The lysosomal enzymes in AF exhibit different activities as pregnancy progresses, as well as at the same stage in different pregnancies.126 Fetal skin becomes impermeable to water127 at about 20 weeks of gestation, when a number of enzymes change in their level of activity, and fetal urine begins to contribute significantly to the AF.128 At some stages of pregnancy, α‐glucosidase has a specific activity in AF exceeding that found in either maternal or fetal serum. This implies a source of these enzymes other than maternal–fetal serum. The disappearance of α‐glucosidase during the second trimester129 may indicate that the fetal liver has assumed a major role in glucose homeostasis. It is now known that this enzyme is of fetal intestinal origin.130, 131
The importance of the developmental biology of enzymes in AF is exemplified by observations made on lysosomal α‐glucosidase, which is deficient in type II glycogenosis (Pompe disease) (see Chapter 21); the initial report indicated that there was no activity of this enzyme in AF from a fetus with Pompe disease.132 Subsequent studies in another pregnancy, however, showed α‐glucosidase activity in AF, whereas cultured AF cells showed no enzyme activity.133 It turns out that the α‐glucosidase in AF is caused by a maltase of fetal intestinal origin,130 distinct from the enzyme deficient in Pompe disease.129
Lysosomal enzyme activities vary in relation to gestational age.134, 135 There is not total concurrence on the observations made about AF lysosomal enzyme
