Medical Definition of Fetoprotein, alpha- (AFP)

Reviewed on 6/3/2021

Fetoprotein, alpha- (AFP): Alpha fetoprotein is a plasma protein that is normally produced by the fetus. It serves as the basis for some valuable tests.

Alpha fetoprotein is manufactured principally in the fetus's liver and gastrointestinal (GI) tract besides the yolk sac, a structure temporarily present during embryonic development.

The alpha fetoprotein level is typically high in the fetus's blood. It decreases in the baby's blood after birth. And by one year of age, it is virtually undetectable.

During pregnancy, alpha fetoprotein crosses the placenta from the fetal circulation and appears in the mother's blood. The AFP level in the mother's blood (the maternal serum alpha fetoprotein) provides a screening test for several disorders including:

The maternal serum alpha fetoprotein (MSAFP) tends to be:

  • High with open neural tube defects such as anencephaly and spina bifida (meningomyelocele); and
  • Low with Down syndrome (trisomy 21, an extra chromosome number 21).

Alpha fetoprotein production is essentially zero after a year of age. However, it increases again under the stimulus of some liver diseases. It may, for example, be produced by viral hepatitis and cirrhosis of the liver. Alpha fetoprotein is also made by primary liver tumors (hepatomas) and by germ cell tumors (teratocarcinoma and embryonal cell carcinomas). A person's serum alpha fetoprotein level can therefore be used to help detect these conditions and monitor their treatment.


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