A

Fetal AFP is synthesized primarily by the yolk sac and fetal liver, so the protein circulates at very high concentrations in the fetus (cord-blood levels ≈10,000–100,000 ng/mL) and then diffuses into amniotic fluid and, in much smaller amounts, across the placenta into maternal blood. Studies measuring all three compartments confirm that fetal serum levels greatly exceed those in amniotic fluid and maternal serum [Van Lith et al., 1991, PMID 1722580; Taketa et al., 1998, PMID 21374462].