of AF
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.
Trace elements
Heavy metals can accumulate in AF, yet their potential impact on the developing fetus is not well understood.356 The topic was reviewed by Caserta et al.,357 who describe their particular concerns with toxicity of lead, mercury, and cadmium on intrauterine growth and neurologic damage. AF copper and zinc are among the trace elements that have stable levels during the second and third trimesters.358, 359 No direct correlation has yet been made in AF studies between central nervous system development or enzymatic reactions and variations in trace element levels in humans. However, zinc deficiency is thought to potentiate the teratogenic effect of alcohol in the fetal alcohol syndrome.360
Further to those observations and another by Chez et al.358 on copper and zinc, Hall et al.343 added proton‐induced X‐ray emission (PIXE) and direct plasma‐atomic emission spectrometry (DCP‐AES) for multi‐element analysis. Their studies were carried out on 90 AF samples obtained between 16 and 19 weeks of gestation from women referred for advanced maternal age (Table 3.3).
Table 3.3 Trace elements in amniotic fluid (see also Table 3.5)
| Element (Z) | N | Mean | SD |
|---|---|---|---|
| B (5) | 88 | 32.2 | 1.7 |
| Mg (12) | 200 | 16.0 a,b | 3.1 |
| Al (13) | 200 | 424.1 | 1.2 |
| Si (14) | 200 | 247.2 | 2.7 |
| P (15) | 200 | 28.3 a,b | 4.0 |
| K (19) | 200 | 148.4 b | 1.1 |
| Ca (20) | 200 | 73.2 a, b | 12.2 |
| Ti (22) | 200 | 13.2 | 2.0 |
| V (23) | 200 | 183.1 | 1.4 |
| Cr (24) | 200 | 4.9 | 1.9 |
| Mn (25) | 200 | 4.7 | 1.8 |
| Fe (26) | 200 | 3475.8 | 14.5 |
| Co (27) | 88 | 44.0 | 1.8 |
| Ni (28) | 200 | 24.0 | 2.2 |
| Cu (29) | 200 | 1437.0 b | 35.3 |
| Zn (30) | 88 | 216.5 | 15.1 |
| Rb (37) | 200 | 217.4 | 80.1 |
| Sr (38) | 200 | 21.2 a | 7.1 |
| Ag (47) | 88 | 15.1 a | 7.8 |
| Sn (50) | 88 | 95.6 | 1.5 |
| Ba (56) | 200 | 17.0 | 6.0 |
| Pb (82) | 88 | 116.7 | 1.4 |
Source: Hall et al. 1983343 (see also Dawson et al. 1999228).
a Concentration mg/mL.
b Arithmetic mean.
Mean gestational age, 17.1 weeks. All concentrations are ng/mL except where noted.
Copper, zinc, bromine, lead, and rubidium assays show no significant differences among groups of normal, hypotrophic, and trisomic fetuses.288 Bussière et al.230 stressed that the wide dispersion of reported metal concentration values in AF may be secondary to sample variability, lack of technical uniformity, and the presence of