What is alpha-fetoprotein screening?

Alpha-fetoprotein screening is a blood test for pregnant women. It can be used to look for a problem in the baby's spine or certain other conditions, such as some chromosome problems.

The test measures the level of a protein called alpha- fetoprotein (AFP) in your blood. AFP is produced by your unborn baby's liver. Some AFP passes from the baby into your bloodstream. If a baby has certain problems, the level of AFP may be abnormally high or low. Sometimes, however, a baby may be normal even though the AFP level is abnormal. Because the test is not completely accurate, your health care provider will usually order other tests if the AFP levels are high or low.

How is the test performed?

A small amount of blood is taken from your arm between the 15th and 18th weeks of your pregnancy. For the test to be interpreted properly, you and your health care provider need to be sure of your due date. Alpha-fetoprotein levels are highest between 15 and 18 weeks of pregnancy. After this time they slowly decrease.

It is important to have the test at the right time in your pregnancy. If the AFP level is higher or lower than normal, enough time must remain in your pregnancy for further steps. These steps might include more tests and counseling.

What conditions does the test find?

A high level of AFP may indicate a problem with the spinal cord, brain, or digestive system. Examples of such problems are:

• spina bifida (the spine has not closed normally)
• anencephaly (the brain or parts of the brain are missing)
• a failure of your baby's abdomen to close, so that the intestines are enclosed in a sac outside the abdomen
• a defect in the esophagus (food pipe).

A low level of the protein may indicate Down's syndrome.

Other possible causes for abnormal levels are:

• an incorrect due date
• the presence of more than one baby
• miscarriage
• a pulling away of the placenta from the uterus
• a baby who is smaller than normal
• an overweight mother, especially if the mother is African American
• diabetes.

How accurate is the test?

The test is not completely accurate. A baby may have a defect even though AFP levels are normal. Or a baby may be quite normal even though AFP levels are abnormal. Usually, if the first blood test shows normal levels, no further tests are done. If the first test shows abnormal levels of AFP, it may be repeated before other tests are done.

For every 1,000 pregnant women tested, about 50 have abnormal test results. Of these 50, just one or two with high AFP levels have abnormal babies. The test finds 90% of babies with anencephaly and 75% with spina bifida.

A new test is now available that provides more accurate screening for birth defects. It is called a triple screen. A sample of your blood is tested for AFP, human chorionic gonadotropin (HCG), and unconjugated estradiol (uE). This test is not yet a standard test and may not be available to everyone.

Who should have this test?

All pregnant women should be offered AFP screening. You should have the test if you:

• have a personal or family history of birth defects
• will be 35 years old or older when the baby is due
• used certain medicines at the time you became pregnant
• have type 1 diabetes.

What happens if the results are abnormal?

Abnormal results indicate the need for further tests, such as:

• ultrasound scans (a scan of your uterus and the baby with sound waves)
• amniocentesis (a test of the fluid around the baby).

In some cases these tests find no reason for abnormal AFP results.

If the results of the AFP test and follow-up tests show that your baby does have a serious problem, your health care provider will talk to you about your choices of treatment.

What are the risks of AFP testing?

There is virtually no risk to you or your baby from this test.

What are the benefits?

Most often, the test reassures you that your baby probably does not have a serious defect.

Abnormal results can help you and your health care provider manage your pregnancy better. For example, if a brain or spinal defect is diagnosed, you and your health care provider can discuss your options. Your health care provider may plan your delivery in a center equipped to deal with these defects, which may improve the outlook for your baby. Your provider can also offer you counseling to help you prepare for the baby's problems before the baby is born.