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alpha-fetoprotein (AFP) (al-fă-fee-toh-proh-teen) n. a protein that is formed in the liver and yolk sac of the fetus and is present in the fetal serum and secondarily in maternal blood. Maternal AFP is used as a marker in prenatal screening tests, performed between the 15th and 20th weeks of pregnancy. Levels are elevated in spina bifida, multiple pregnancies, and certain other conditions and decreased in Down's syndrome.