The amnion regulates movement of fetally derived alpha-fetoprotein into maternal blood.

The present investigation documents that, under normal conditions, most fetally produced AFP reaches the maternal circulation via diffusion across the amnion from amniotic fluid. This has been determined by comparing maternal serum AFP levels with amniotic fluid albumin concentrations in paired samples. The proportionally demonstrated between them indicates a proportional, transamniotic exchange of the two proteins, each originating on opposite sides of the amnion. Albumin is known to reach amniotic fluid by transamniotic diffusion from maternal blood. All amnions restrict AFP movement into maternal serum, but some are distinctly more restrictive than others; in such cases, a relatively greater increase in amniotic fluid AFP concentration would likely have to occur from a fetal lesion before being reflected in maternal serum. Inconsistencies found in several paired samples identify that other variables may also influence passage of AFP to the mother.