- Last Update On : 2013-02-17
Alpha fetoprotein (AFP) is a 69, 000 kD single chain polypeptide which is similar in size and structure to human serum albumin. In human embryos, AFP is first made in the yolk sac and later in the fetal liver. As the fetal liver matures, it gradually switches to albumin synthesis. In the fetus, AFP synthesis gradually declines to the normal adult range by 6 to 12 months after birth. AFP passes by diffusion from the fetus through fetal glomeruli into the fetal urine and then into the amniotic fluid. Amniotic levels peak in the first trimester and are 100 times lower than fetal plasma levels.
AFP appears in maternal serum by transplacental transfer. During pregnancy, maternal serum AFP levels rise steadily to a peak of approximately 500 ng/mL at about 32 weeks of gestation. Thereafter, they decline until term.
Screening of pregnant women for increased AFP serum levels is a routine method of detecting neural tube defects of the fetus prior to birth. Neural tube defects (NTD), such as spina bifida and anencephaly, result from the incomplete closure or functional incompetence of the neural tube. This results in leakage of fetal AFP into the amniotic fluid and increased levels in maternal sera. Screening for NTD is most useful and accurate when performed between 15 - 20 weeks gestation because there is a greater spread in normal values with advancing pregnancy. A screening program detects approximately 88% of fetuses with anencephaly and 77% of those with open spina bifida. Increased levels are also seen with multiple pregnancy, fetal distress and intrauterine death.
Decreased levels of AFP are associated with Down Syndrome. Prenatal AFP screening detects 20% of affected fetuses in women less than 35 years of age. Abnormal AFP levels are not diagnostic for neural tube defect or Down Syndrome, but define a population that requires further testing. The reference range is 0.25 - 2.50 MoM (multiples of the median) and a predicted risk for neural tube defect of Down Syndrome <1/270.
Specimen requirement is one SST tube of blood.