Alpha-fetoprotein is a fetal plasma protein, which is like the adult plasma protein, albumin. Fetuses contain a great deal of AFP and in fact, the level of AFP in the fetal blood is 1000 times that of the maternal serum level. It takes only a few drops of fetal serum or blood to leak into the maternal circulation for the maternal levels of AFP to increase to abnormal levels. In normal pregnancies most, if not all, of the AFP enters the maternal circulation via the amniotic fluid. Fetuses excrete AFP into their urine and this becomes part of the amniotic fluid.
