Alpha-foetoprotein (AFP) is the major foetal plasma protein. It is produced from a gene on chromosome 4 and is homologous to adult albumin. It is made initially by the yolk sac and then by the liver.
The concentration of AFP peaks in the foetal bloodstream at 2-3 g/l around 12-14 weeks of gestation and then falls. AFP passes into the amniotic fluid where it is found at 1% of the concentration in foetal plasma.
AFP passes into the maternal bloodstream where it may be detected by immunoradiometric assay.
Malformations such as neural tube defects elevate maternal serum and amniotic fluid AFP by leakage from exposed foetal capillaries.
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