Concanavalin A variants of alpha-fetoprotein in first trimester fetuses with trisomy 21 and with normal karyotypes.

Total alpha-fetoprotein (AFP) concentrations and proportions of AFP non-reactive with the lectin concanavalin A (Con A) were studied in extracellular fluid of 22 first-trimester fetuses. Total AFP concentrations were significantly lower in fetuses with Down's syndrome than in those with Mendelian-inherited diseases and normal karyotypes. The proportion of non-Con-A-reactive AFP was low in the fetal compartment (< or = 3.5%) irrespective of the fetal karyotype. So the AFP production in the fetal liver as well as in the yolk sac seems to be reduced in Down's syndrome. The fetus itself contributes only marginally to the non-Con-A-reactive AFP pool of amniotic fluid, which is therefore almost entirely of yolk sac origin. This pool is preserved well into the second trimester of pregnancy, despite cessation of yolk sac AFP production at the end of the first trimester, indicating a very slow rate of disappearance of proteins out of amniotic fluid.