AFP is manufactured principally in the fetus's liver and, also, in the fetal gastrointestinal (GI) tract and the yolk sac. The level of AFP is usually high in the fetus's blood, goes down in the baby's blood after birth, and by age one 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 MSAFP) provides a screening test for a number of disorders including:
- Open neural tube defects (anencephaly and spina bifida)
- Down syndrome (and other chromosome abnormalities)
- High with open neural tube defects such as anencephaly and spina bifida (meningomyelocele)
- Low with Down syndrome (trisomy 21, an extra chromosome number 21)