Special tests can give information about a pregnant woman's risk of having a baby with certain birth defects, such as Down syndrome or spina bifida. One of these is a blood test called a maternal serum screening test. It tests for products from the pregnancy that are also in the woman's blood (serum). Your doctor may offer you a maternal serum screening test. The decision to have one of these tests is up to you.

Screening Tests
A screening test is a test that is done when there are no symptoms or known risk factors present. It is not a diagnostic test.

Birth Defects
A birth defect is a physical problem that a baby has at birth. Most babies — 97 percent — are born without major birth defects.

Some birth defects may be inherited (passed down) from the parents. Others may result from a virus or being exposed to a substance, such as alcohol, during pregnancy. Most often, the reason for the defect is unknown.

Maternal serum screening tests may help to detect a number of types of birth defects. But, there are birth defects that these tests will not find.

Open Fetal Defects
With an open fetal defect, part of the body of the fetus has not developed a skin covering. Open fetal defects occur most often in the abdominal wall or around the spine.

The most common open fetal defects are open neural tube defects (ONTDs). With an ONTD, the fetus's brain, spinal cord or their coverings do not form as they should.

Spina bifida (sometimes called "open spine") is one type of ONTD.

Anencephaly is another type of ONTD. It occurs when the brain and head do not develop normally.

ONTDs occur in about one to two in 1,000 births.

Down Syndrome
Instead of two number 21 chromosomes, persons with Down syndrome have three. This is called trisomy 21. Down syndrome causes mental retardation to varying degrees.

Down syndrome is one of the most common problems that can happen in the chromosomes of the fetus. It occurs in about one in 800 births. The risk of having a live baby with Down syndrome increases with the woman's age. The risk is only one in 1,667 for a 20-year-old woman. By age 35, it has increased to one in 378.

Who Should Be Tested
Maternal serum screening tests can find a higher-than-average risk of ONTDs and Down syndrome.

Maternal Serum Screening Tests
Alpha-Fetoprotein Test
Alpha-fetoprotein (AFP) is a protein produced by a growing fetus. It is present in amniotic fluid, fetal blood and, in smaller amounts, in the woman's blood.

The AFP test is done at 15 to 18 weeks of pregnancy in most cases. This is when the test is most accurate.

At 15 to 18 weeks, AFP levels are higher than normal in the maternal serum of many women (80 percent) carrying fetuses with open fetal defects.

AFP levels are lower than normal in most Down syndrome fetuses.

Multiple Marker Screening Tests
Adding certain tests to the AFP test can give more information about your risk of having a baby with Down syndrome than the AFP test alone. These are called multiple marker screening (MMS) tests. The best combination of tests is not yet known. Many doctors use two or three tests together.

What Do the Test Results Mean?
If AFP levels are normal or if your MMS results are normal, your risk of having a baby with either an open fetal defect or Down syndrome is low. There is still a chance, though, that the risk of a defect in your baby was not detected by the screening test.

Risk of Open Fetal Defects
Your baby may be at higher-than-average risk for open fetal defects if your AFP levels are higher than average. Your doctor may offer you diagnostic tests to find the reason for a high level of AFP.

The first test you may be offered is an ultrasound exam.

If no reason for high AFP levels is found with ultrasound, you may be offered amniocentesis to measure AFP in the amniotic fluid.

Risk of Down Syndrome
You are at increased risk for having a baby with Down syndrome if your AFP levels are lower than normal. If you had MMS tests and your AFP and estriol levels are lower than normal and your hCG levels are higher than normal, you also may be at higher risk.

False Results
No test is perfect. Not every abnormal result of a screening test will mean that your baby has a birth defect. An abnormal result may mean that the fetus was older or younger than thought or that there is more than one fetus.

Sometimes there is no reason for an abnormal screening test, and amniocentesis will show that your baby does not have the birth defect.

Not every normal result of a screening test will mean that your baby does not have a birth defect. Not all cases of open fetal defects or Down syndrome can be found.

Finally...

Maternal serum screening tests can give you an idea of your risk for having a baby with birth defects. A normal test result does not guarantee that your child will not have birth defects. It may reassure you that the risk is not increased, though. An abnormal test result does not always mean that your child will have birth defects. You should think about further testing, though.

This excerpt from ACOG's Patient Education Pamphlet is provided for your information. It is not medical advice and should not be relied upon as a substitute for visiting your doctor. If you need medical care, have any questions, or wish to receive the full text of this Patient Education Pamphlet, please contact your obstetrician-gynecologist.