A plasma protein normally produced by a fetus, principally in the fetus’s liver, the fetal gastrointestinal tract, and the yolk sac, a structure temporarily present during embryonic development. Abbreviated AFP. The level of AFP is typically high in the fetus’s blood. It goes down after birth. By 1 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 pro- vides an opportunity to screen for a number of dis- orders, including open neural tube defects (such as anencephaly and spina bifida), Down syndrome, and other chromosome abnormalities.