ALPHA-FETOPROTEIN IN THE AMNIOTIC FLUID

• a-fetoprotein is a glycoprotein that is initially synthesized in the embryonic yolk sac and subsequently in the fetal liver and gastrointestinal tract.
• It is present in high concentrations in fetal plasma.
• Function:
• Unknown.
• ? functions similar to albumin in the adult (maintains oncotic pressure, transfers and binds various plasma constituents).
• Molecule enters the amniotic fluid via the fetal circulation, gastrointestinal secretions and transudation from exposed blood vessels.
• In normal pregnancies there is always some leakage of a-fetoprotein into both amniotic fluid and maternal circulation.
• Peak levels are at 13 weeks and then it slowly decreases.
• In neural tube defects the levels of a-fetoprotein do not increase but its concentration in amniotic fluid increases and protein leaches across the membrane into maternal circulation.
• In the graph below note that there is less overlap between normal fetuses and fetuses with open neural tube defects when compared to maternal serum AFP levels.
• False positives (green shaded area of the graph) can be excluded by the presence of a normal amniotic fluid n-acetylcholinesterase level.

Amniotic fluid AFP levels at 16-18 weeks of gestation