Mechanisms by which amniotic-fluid alpha-fetoprotein may be increased in fetal abnormalities.

Assuming that urination is the principle route by which alpha-fetoprotein (A.F.P.) passes into the amniotic fluid from the normal fetus, and that A.F.P. is mainly degraded by a process of fetal swallowing and digestion, 4 possible mechanisms by which amniotic-fluid A.F.P. is increased by fetal abnormalities are postulated.(1) Amniotic-fluid A.F.P. is increased in open neural-tube defects by leakage of A.F.P. from fetal serum and cerebrospinal fluid.(2) In exomphalos exposure of blood-vessels in the extruding viscera permits transudation of A.F.P. into the amniotic fluid.(3) In nephrosis, fetal proteinuria increases amniotic-fluid A.F.P.(4) Impaired fetal swallowing or digestion would account for increased amniotic-fluid A.F.P. in congenital malformations of the orogastrointestinal tract.