Definition: A protein made in the liver of a developing fetus during pregnancy. This protein is found in the fluid surrounding the fetus (amniotic fluid) and in the mother’s bloodstream. Testing the mother’s blood levels of AFP can provide information about how the developing baby is doing. Unexpected AFP levels may suggest that the baby was conceived earlier or later than originally thought, the possibility of a pregnancy with multiples (twins or triplets), or the possibility of birth defects.