Chapter 14 - Amniotic Fluid

# Physiology

## Function

* Amniotic fluid is present in the amnion, a membranous sac that surrounds the fetus.
* It acts as a cushion for the fetus, allows fetal movement, stablizes the temperature, permits proper lung development and allows exchanges of water and chemicals to take place between the fluid, the fetus and the maternal circulation (intramembranous flow).


## Volume

* Amniotic fluid volume is regulated by a balance between the production of fetal urine and lung fluid and the absorption from fetal swallowing and intramembranous flow.
* The amount of amniotic fluid increases throughout pregnancy.
* During the first trimester, approximately 35 mL of amniotic fluid is derived primarily from the maternal circulation.
* After the first trimester, fetal urine is the major contributor to the amniotic fluid volume and it is regulated by fetal swallowing.
* Failure to swallow results in accumulation (polyhydramnios) and is an indication of fetal distress.
* It is often associated with neural tube disorders, fetal structural anomalies, cardiac arrhythmias, congenital infections or chromosomal abnormalities.
* Increased fetal swallowing, urinary tract deformities and membrane leakage are possible causes of decreased amniotic fluid (oligohydramnios).
* Oligohydramnios may be associated with umbilical cord compression, resulting in decelerated heart rate and fetal death.
* During the latter third to half of pregnancy, the fetus secretes a volume of lung liquid necessary to expand the lungs with growth.
* During each episode of fetal breathing movement, secreted lung liquid enters the amniotic fluid, as evidenced by lung surfactants that serve as an index of fetal lung maturity.
* During the third trimester, amniotic fluid reaches a peak volume of 1 L.

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## Chemical Composition

* Amniotic fluid has a composition similar to that of the maternal plasma and contains a small amount of sloughed fetal cells from the skin, the digestive system and the urinary tract.
* The fluid also contains biochemical substances that are produced by the fetus, such as bilirubin, lipids, enzymes, electrolytes, nitrogenous compounds and proteins.
* The chemical composition of the amniotic fluid changes when fetal urine production begins.
* The concentrations of creatinine, urea and uric acid increase, whereas glucose and protein concentrations decrease.
* Concentrations of electrolytes, enzymes, hormones, and metabolic end products also vary but are of little clinical significance.

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## Maternal Urine vs Amniotic Fluid

| | Maternal Urine | Amniotic Fluid |
|----|----|----|
| **Creatinine** | >3.5 mg/dL, as high as 10 mg/dL |