Screening for Fetal Abnormalities

The alphafetoprotein (AFP) assay is one of many screening tests that are used to detect fetal problems. AFP is a protein that the fetal liver produces. If there are any openings in the fetal skin, an unusually large amount of alphafetoprotein escapes into the amniotic fluid and crosses the placenta to the mother's blood stream.

The AFP assay measures the concentration of certain hormones in the mother's blood to predict whether the fetus is likely to have a neural tube defect or a chromosomal abnormality. Neural tube defects are the most common fetal abnormalities that result in high alphafetoprotein. These abnormalities arise when the neural tube, destined to become the spine, fails to close properly, and leaves an opening. If the opening is at the base of the spine, the defect is called spina bifida, which generally results in paralysis of the lower body. If the defect occurs at the upper end of the neural tube, brain abnormalities often associated with significant retardation can be expected.

Down's syndrome is an example of a chromosomal problem that results from an extra copy of chromosome 21. For reasons that are unclear, the babies of women who have abnormally low levels of AFP are more prone toward this condition than women with higher levels of AFP Doctors made this unexpected finding when routine testing for neural tube defects was first instituted. After careful study, doctors concluded that AFP screening could be used routinely to detect Down's syndrome and other, similar chromosomal problems.

In the years since the test was introduced, several modifications have been made to it. To increase the test's accuracy, other hormones are also measured. In the AFP 2 test, human chorionic gonadotropin (HCG) is measured, and in the AFP 3 test, estriol is also measured. Both of these hormones are associated with the placenta. Fetuses afflicted with Down's syndrome tend to produce abnormal levels of these hormones as well as unusually low levels of alphafetoprotein.
It is important to remember that the AFP assay is a screening test, not a diagnosis. It can only predict the likelihood that a particular fetus has a neural tube defect or a chromosomal problem by comparing one test's results with thousands of other results. That's why the AFP assay does not report a diagnosis; it offers a probability.

Next Chapter: Amniocentesis