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alpha-fetoprotein or à-fetoprotein or chiefly Brit alpha-foetoprotein or à-foetoprotein n a fetal blood protein present abnormally in adults with some forms of cancer (as of the liver) and normally in the amniotic fluid of pregnant women but with very low levels tending to be associated with Down syndrome in the fetus and very high levels with neural tube defects (as spina bifida) in which the tube remains open

a protein that is formed in the liver and yolk sac of the fetus and is present in the amniotic fluid and secondarily in maternal blood. The level of afp can be detected by a maternal blood test performed between the 16th and 18th weeks of pregnancy to aid prenatal diagnosis of certain fetal conditions. Levels are elevated in open neural tube defects (e.g. spina bifida), twins and triplets, open abdominal wall defects (e.g. gastroschisis), and fetal death. Levels of afp are decreased in Down's syndrome. However, afp levels are affected by the length of gestation and the mother's weight, and these factors must be considered when interpreting the results. When levels are unexpectedly high or low, further investigations (for example ultrasound scanning) are indicated.

Alpha-fetoprotein is also produced by certain tumours (see tumour marker).