Throughout intrauterine development, the fetus exists in a compartment filled with amniotic fluid, allowing free movement. This fluid acts as a protective cushion against potential injuries, helps maintain a stable temperature, and is inhaled and swallowed by the fetus, contributing to normal lung development. Amniotic fluid is a dynamic substance, with its volume and chemical makeup regulated within narrow limits. The amount of amniotic fluid gradually increases until approximately 34 weeks of gestation, after which it slightly decreases until the 40th week and then declines more sharply until the 42nd week.
The volume at any given moment is a function of several interrelated fluid fluxes. In the early stages of pregnancy, the composition of amniotic fluid resembles a complex dialysate derived from maternal serum. As the fetus grows, changes occur in the fluid's composition; notably, sodium concentration and osmolality decrease while urea, creatinine, and uric acid levels increase.
Amniotic fluid also contains various steroid and protein hormones. Initially, it has little to no particulate matter, but by 16 weeks of gestation, a significant number of cells shed from the amnion, skin, and tracheobronchial tree are present. These cells are crucial for antenatal diagnosis and serve as a source of DNA for karyotype analysis following amniocentesis.
Typically, fetuses do not defecate during pregnancy; however, if under severe stress, they may pass meconium. This material contains bile pigments that can stain the amniotic fluid green, indicating fetal stress.
The amniotic fluid index is a standardized method used to measure the amount of amniotic fluid during pregnancy. This measurement helps evaluate fetal well-being and detect potential complications such as oligohydramnios or polyhydramnios. The amniotic fluid index is typically performed on patients who are at least 24 weeks pregnant with a singleton gestation.
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