Читать книгу Genetic Disorders and the Fetus - Группа авторов - Страница 90

Fluid exchange between the fetus and the mother occurs via several routes and through different mechanisms, and varies throughout pregnancy. Large volumes of fluid are transferred across the fetal membranes, which are made up of five layers of amnion and four layers of chorion.³ Electron microscopy of the amnion has revealed a complex system of tiny intracellular canals that are connected to the intercellular canalicular system and the base of the cell.⁴ Studies in primates suggest that the AF is a transudate of the maternal plasma and becomes like other fetal fluids in the presence of the fetus, which contributes urine and other body secretions to the AF.⁵

Osmotic or diffusion permeability, hydrostatic pressure, chemical gradients, and other mechanisms are responsible for the fluid exchange between fetus and mother.⁶ In normal pregnancies, intra‐amniotic pressure at 16 weeks ranges between 1 and 14 mmHg.⁷ Fisk et al.⁸ studied AF pressure from 7 to 38 weeks and found that it increased with gestational age and may be determined by anatomic and hormonal influences or gravid uterine musculature, but was not influenced by the deepest vertical pool, AF index, maternal age, parity, gravity, fetal sex, twinning, or time of delivery. These authors suggested that AF pressure did not change significantly after removing fluid samples in early or late amniocentesis.⁹ During the second trimester, total AF turnover is complete within about 3 hours.¹⁰ About 20 mL of AF/hour is swallowed by the fetus; that is, approximately 500 mL/day.¹¹ At term, the exchange rate between fetus and mother may approach 500 mL/h.^{10, 12}

Although the fetus depends largely on the placenta for nutrient transport, it is also protected from marked fluctuations in maternal metabolism. The increase of creatinine, α‐glutamyl transferase, and β₂‐microglobulin concentrations in AF after 10 weeks confirms the maturation of fetal glomerular function and reflects the fetal kidney development from the mesonephros to the metanephros.¹³ Active renal function is evident from the ability of the fetal kidney to concentrate radiopaque substances given intravenously to the mother, thereby allowing visualization of a fetal pyelogram.¹⁴ AF is mainly produced by the fetal kidney as pregnancy progresses and oligohydramnios may reflect renal structural anomalies, impaired swallowing, placental pathology, or general growth restriction.¹⁵