Alpha-Fetoprotein Blood Test: Screening for Birth Defects

Page content

Why the Alpha-Fetoprotein Blood Test is Used

Birth defects impact not only the child with the defect, but the entire family as well. Screening for birth defects early in pregnancy — especially in women who are at high risk of having a baby with a birth defect — helps the family prepare well in advance of the birth for an infant who may have special needs.

In the past, the only way to screen for birth defects before birth was by performing invasive procedures such as amniocentesis and chorionic villus sampling (CVS). These tests effectively detect chromosomal abnormalities that can cause birth defects, but they also increase the risk of miscarriage. Ultrasound imaging can detect some defects in a developing fetus, but it is useful for health practitioners to know whether an expensive, highly detailed ultrasound is indicated to check for specific birth defects.

The alpha-fetoprotein (AFP) blood test is a test done on the pregnant woman. In a normal pregnacy, alpha-fetoprotein is present in the mother’s blood in a specific amount. Babies with certain birth defects will cause the mother’s AFP level to differ from the normal amount. This blood test, called maternal serum alpha-fetoprotein test or MSAFP, is therefore an excellent screening tool that is harmless to the fetus, though the test cannot provide a diagnosis.

What Does High Alpha-Fetoprotein Mean?

An elevated maternal serum alpha-fetoprotein level can mean a number of things. This uncertainty is why MSAFP is a screening tool, not a diagnostic tool. It often means simply that the pregnancy is dated wrong, because the gestational age of the fetus determines the normal AFP level. Less frequently, it may indicate a serious condition, such as neural tube defect (spina bifida or anencephaly), hernia (such as gastroschisis, a condition in which the baby’s intestines are located on the outside of the body), a blockage, or a tumor in the mother or the fetus. On the other hand, high AFP may be “idiopathic” — normal for that pregnancy and not indicating a problem. In any event, high AFP requires follow-up to determine the cause, if any.

What Does Low Alpha-Fetoprotein Mean?

A low maternal serum alpha-fetoprotein level most often indicates a chromosomal abnormality, such as Down syndrome (trisomy 21) or the more serious Edwards syndrome (trisomy 18). As with elevated serum AFP, it could also simply indicate incorrect dating of the pregnancy, so it should be viewed as a cause for further testing, not panic.

Keeping Maternal Serum Alpha-Fetoprotein Results in Perspective

Maternal serum AFP is intended only to screen for a higher risk of birth defects. According to the American Pregnancy Association, only 1 in 16 to 1 in 33 pregnancies with an abnormal MSAFP result actually are due to a birth defect in the fetus; that means that most abnormal MSAFP results are not due to birth defects. An abnormal alpha-fetoprotein result is only an indication that more testing is required, not necessarily an indication of a serious problem.