Maternal serum alpha-fetoprotein screening

Alpha-fetoprotein is a glycoprotein with a molecular weight of about 70,000 that is synthesized first by the embryonic yolk sac and later by the fetal gastrointestinal tract and liver. This major fetal protein reaches its highest concentration in fetal serum and amniotic fluid at about 13 weeks' gestation. Its concentration in fetal serum (measured in milligrams) is approximately 150 times that found in amniotic fluid (measured in micrograms), and an even smaller amount enters maternal blood (measured in nanograms), most likely through placental membranes and decidua. Perhaps somewhat paradoxically, although alpha-fetoprotein concentrations in both fetal serum and amniotic fluid decline.