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Various nonenzymatic constituents and characteristics of AF are listed in Table 3.2. Elevated values of biochemical constituents in AF may be nonspecific, such as 5‐hydroxyindole‐3‐acetic acid in a fetus with Cornelia de Lange syndrome³⁵³ or acid‐soluble glycoproteins in AF in CF.²⁶¹ Reduced amounts of 5‐hydroxyindole‐3‐acetic acid,²⁷¹ and increased amino acids (especially methionine, isoleucine, leucine, tyrosine, and phenylalanine) have been observed in the AF of fetuses with NTDs.³⁵⁴ Two likely reasons are transudation across the defect and a nonspecific reflection of fetal distress with hypoxia. Certain other constituents of cell‐free AF, such as 17‐ketosteroids, are especially relevant to prenatal diagnosis.

Table 3.2 Biochemical constituents and other characteristics of amniotic fluid (see text for additional sources)

Biochemical constituent/characteristic	Selected reference(s)
Acetylcholine receptor	197
Acetylcholinesterase (AChE)	198
Acid–base	199, 200
ADAM‐8	5, 201
Adrenomedullin	202
α1‐Antitrypsin	205
α‐Fetoprotein	this chapter
α1‐Macroglobulin	206
Albumin	203
Alkaline phosphatase	189, 204
Amino acids	149, 207
Amylase	208
Androgens	209
Angiogenin	210
Anticardiolipin antibodies	211
Antithrombin	206
Antiviral effect	212
Apolipoprotein	213
Apolipoprotein A	214
Arginosuccinic acid	215
Arylsulfatase A	216
Atrial natriuretic factor	217
Bacteria	218
Bacterial growth, inhibitory effect	219, 220
Basement membrane protein	67
β‐Endorphin	221
β‐Hydroxybutyrate	222
β2‐Microglobulin	223
Bile pigments	183
Bilirubin	224
Blood group substances	225, 226
Blood urea nitrogen	227
Cadmium	228
Candida albicans	229
Calcium	230
Calgranulins	231
Carcinoembryonic antigen	232
Carnitine	233
Catecholamines	234
Chemokines	235
Ceruloplasmin	203
Cholesterol	89
Chorionic gonadotropin	209
Chorionic somatomammotropin	209
Citric acid	236
Citrulline	215
Clara cell protein	153
Complement	203
Copper	238
Cortisol	239
C‐reactive protein	237
Creatine phosphokinase	119
Creatinine	240
Cystatin C	241, 242
Cytokines	235, 243
Cytomegalovirus	244
Decidua‐associated protein	245
Defensins	246
7‐Dehydrocholesterol	247
C‐reactive protein	237
Disaccharidases	131
Electrolytes	248
Endothelin	238
Estrogens	209
Exosomes	239, 249
Factor IX	71
Fatty acids	250
Ferritin	251
Fibronectin	252, 253
Follicle‐stimulating hormone (FSH)	254
Fractalkine	255
Free fatty acids	103
Fucose	256
Fumarylacetoacetate	257
γ‐Glutamyl transferase	258
Globulin	68
Globulin, cold‐insoluble	259
Glucagon	260
Glucose	237
Glycoproteins	261
Glycosaminoglycans	69
Growth hormone	262
Growth‐inhibiting property	263
Heat shock protein	264, 265
Hemopexin	266
HLA‐G isoforms	267
Homocysteine	147
Human leukocyte antigen G	268
Human leukocyte antigens (HLA)	269
Hydrocarbons	89
2‐Hydroxybutyric acid	270
5‐Hydroxyindole‐3‐acetic acid	271
Hydroxyproline	272
Immunoglobulin	273, 274
Inhibin (utero)	274
Insulin	260, 276
Insulin‐like growth factor binding protein‐1	275
Intercellular adhesion molecule	277
Interferon	278
Interleukins	253, 279
Iodide	280, 281
Iron	230
Isoferritin	282
Isomylases	283
Isoprostanes	284
Kallikrein	285
Lactate dehydrogenase	286
Lactoferrin	287
Lead	288
Lecithin/sphingomyelin (L/S) ratio	204
Lipids	89
Lipopolysaccharide‐binding protein	90
Luteinizing hormone (LH)	255
Lysosomal enzymes	131
Lysozyme	289
Macroglobulin	206
Macrophage colony‐stimulating factor	290
Magnesium	230
Manganese	230
Meconium	291
Metalloproteinase	292, 293
Metals	228
Methylcitrate	215
Methylmalonic acid	215
Micelles	294
Muramidase (see lysozyme)	–
Mycoplasma	295
Neuraminic acid	296
Neurotrophins	297
Neuron‐specific enolase	298
Neutrophil elastase	286
Nitric oxide	299, 300
Nucleic acids	301
Nucleosome	302
Nicotine	303
Oligosaccharides	304
Organic acids	305, 306
Osmolality	307
Orosomucoid	76
Oxalate	308
Oxygen tension	309
Oxytocin	310
Palmitic acid	204
PAPP‐A	311
Peptidase activity	188
Peroxidase	312
Phospholipids	313, 314
Phospholipase	315
Phytoestrogens	316
Placental growth factor	317
Plasminogen	318
Plasminogen activator	319
Polypeptides	320
Procoagulant	321
Procollagen propeptides	322
Progesterone	209
Prolactin	323
Prostaglandin	324
Protein	325
Protein‐bound iodine	326
Protein C and S	327
Proteoglycan	328
Psoriasin	329
Prothrombin	71
Pyridoxine oxalate	330
Pyruvate kinase	123
Renin	330
Rubella	331
Rubidium	288
Secretor typing	332
Selenium	228
Sialic acid	333
Sodium	334
Somatomedin	335
Spectrophotometry	336
Steroid hormones	209
Stromelysins	337
Succinylacetone	162
Surface‐active material	338
Surface tension	339
Tamm–Horsfall glycoprotein	68
Testosterone	340
Thromboplastin	321, 341
Thrombopoietin	342
Trace elements	343
Transferrin	51, 203
Triglyceride	89
3.3′,5′‐Triiodothyronine	344
Troponin G	345
Trypsin	185
Tumor necrosis factor	346
Tyrosine kinase	347
Ureaplasma urealyticum	295
Uric acid	348
Urinary trypsin inhibitor	349
Vitamin A	350
Vitamin B12	351
Vitamin D	352
Viral antibodies	273
Viruses	273
Volume of AF	27, 28
Zinc	228, 230

For enzymes, see Chapters 21, 23, 24, and 25.

The insulin concentration increases in the AF of healthy, pregnant women,²⁷⁶ with a biphasic course between 16 and 42 weeks and a zenith at the 30th week.³⁵⁴ In women receiving glucocorticoids or betamimetics, or in fetuses with high glucose levels, the insulin levels may show a twofold increase, whereas very low levels are found in placental insufficiency and other conditions leading to fetal distress.

Thirteen major polypeptides, of which five had not been identified previously, were revealed by two different techniques. Their molecular weights ranged from 11 to 220 kDa. Prado et al.³²⁰ suggest that those polypeptides might serve as useful references in molecular studies of fetal pathology. Maternal serum relaxin during pregnancy maintains the myometrial quiescence and facilitates uterine stroma remodeling during uterine growth. In AF, this protein³⁵⁵ rises from 58 ng/L at 10 weeks to 142 ng/L at 14 weeks and declines to 55 ng/L at 22 weeks. Relaxin may be derived from the decidualized endometrium rather than the maternal circulation, from placental transfer or fetal synthesis of this peptide.