AFP Test For a Birth Defect

What is the AFP Test?

The AFP test is a blood test available early in pregnancy to help identify women who may be carrying a fetus (unborn baby) with certain types of birth defects.

What is AFP?

AFP (Alpha-Fetoprotein) is a substance formed by the growing baby. Some AFP is normally passed into the amniotic fluid and into the mother's blood.

The amount of AFP in the amniotic fluid and the mother's blood is usually small; however, in certain situations there may be more or less AFP than usual. Causes for increased amount, of AFP include: 1) twins, 2) mistaken due date and 3) a neural; tube defect in the unborn baby Causes for a decreased amount of AFP include: 1) mistaken due date or 2) may indicate the occurrence of Down Syndrome (Trisomy 21) in the pregnancy.

What are Neural Tube Defects?

Neural tube defects are a group of birth defects affecting the development of the brain and spinal cord. This is seen in approximately one to two per thousand births. The cause is not known. One form is called anencephaly in which the brain and skull are not fully developed. This is not compatible with life beyond a few days.

The other major form of neural tube defect is Spina Bifida (open spine). Spina Bifida involves an opening somewhere in the spine and affects the development of the spinal nerves. This generally causes weakness or paralysis of the legs, problems with bowel and bladder control and hydrocephalus (water on the brain).

The severity of Spina Bifida can vary. Surgery is required soon after birth and special medical care is necessary throughout life. In the least severe forms, the handicaps may be minimal and as adults these patients may lead a self-sufficient life. However, most patients have many hospitalizations, a shortened lifespan and varying degrees of mental retardation.

What is Involved In The Test?

Women who request AFP screening have a blood test done between the 16th and 18th week of pregnancy, in which a small amount of blood is taken from the arm. For most women (97 out of 100) the blood test will show normal levels of AFP and no further tests are necessary.

What Does it Mean if the AFP Test is Positive?

Some women (3 out of 100) will have an increased amount of AFP and some (1 or 2 out of 100) will have a low levels of AFP.

IF THE AFP IS EITHER ELEVATED OR LOW IN THE FIRST BLOOD TEST, THIS DOES NOT MEAN THAT THE BABY HAS A NEURAL TUBE DEFECT OR OTHER PROBLEMS. THIS IS ONLY THE FIRST STEP IN A SEQUENCE OF TESTS THAT MAY BE DONE.

When the first blood test shows either a high or low AFP level, there is usually a repeat blood test. Often this second test shows normal levels of AFP and no further tests are necessary.

However, if the second test also shows increased levels of AFP, than an ultrasound (sonogram) examination will be indicated. Ultrasound is a technique that utilizes sound waves to visualize the fetus. Ultrasound may detect two normal causes for an elevated AFP level: 1) twins, or 2) a pregnancy farther along than originally estimated. If either of these situations is detected, your health care provider can provide for better care during pregnancy. Low AFP levels may indicate a pregnancy is not as far along as originally thought.

If the ultrasound does not explain high or low AFP, then the woman will be offered amniocentesis. Amniocentesis is a procedure that involves withdrawing a small amount of amniotic fluid which surrounds the baby. Higher than normal levels of AFP can be detected in the amniotic fluid and will accurately diagnose most cases of neural tube defect. Only one out of every ten amniotic fluid samples analyzed this way have an elevated AFP level indicating neural tube defect. Chromosome study from the amniotic fluid cells can accurately diagnose Down Syndrome and other problems of chromosomal abnormality.

When a neural tube defect is detected the woman can make arrangements for special medical care of the newborn baby or she may decide to terminate the pregnancy.

What does it Mean if the AFP Levels are Normal?

The AFP test can detect almost all cases on anencephaly and most cases of Spina Bifida as well as a few other less common types of birth defects. However, most types of birth defects cannot be detected in this manner. Some cases of Spina Bifida are completely skin covered and do not leak excess AFP. These occasional cases cannot be detected and only one out of five pregnancies with Down Syndrome will have low a AFP level.

Who Should Have the AFP Test?

You may wish to pursue this test if you or your baby's father have a family history of a neural tube defect. There may be additional reasons for your health care provider to suggest this test or you may wish to gain more information about your pregnancy. Please feel free to discuss your questions with your health care provider.