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AFP is a plasma protein that is normally produced by the fetus. It serves as the basis for some valuable tests. AFP is manufactured principally in the fetus's liver and, also, in the fetal gastrointestinal (GI) tract and the yolk sac, a structure temporarily present during embryonic development. The level of AFP is typically high in the fetus's blood. It goes down in the baby's blood after birth. And by a year of age, it is virtually undetectable. During pregnancy, AFP crosses the placenta from the fetal circulation and appears in the mother's blood. The level of AFP in the mother's blood (the maternal serum AFP) provides a screening test for a number of disorders including: Open neural tube defects (anencephaly and spina bifida); and Down syndrome (and other chromosome abnormalities). The maternal serum AFP (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). AFP production is essentially nil after a year of age. However, it starts up again under the stimulus of some diseases in the liver. It may, for example, be produced by the liver in viral hepatitis and cirrhosis of the liver. AFP is also made by primary liver tumors (hepatomas) and by germ cell tumors (teratocarcinoma and embryonal cell carcinomas). A person's serum AFP level can therefore be used to help detect these conditions and monitor their treatment.