Maternal Blood Screening Tests during Pregnancy

What are maternal blood screening tests?

Maternal blood screening tests are a way to check the risk of birth defects before a baby is born. The tests measure the levels of 3 or 4 substances in a sample of the mother's blood during pregnancy. Common names for these tests are serum screening tests, triple screen, and quad screen.

Substances that may be measured in these tests are:

• alfa-fetoprotein (AFP), a protein produced by the baby's liver
• human chorionic gonadotropin (hCG), a hormone produced by the placenta
• estriol (uE3), a hormone produced in the placenta and the liver of the baby.
• dimeric inhibin-A (DIA), another protein made by the placenta.

Why are these tests done?

Some of the birth defects that might be detected by these tests are:

• chromosome abnormalities
• Down syndrome
• brain or spinal cord defects (also called neural tube defects), such as the problems called spina bifida (the spine has not closed normally) and anencephaly (all or part of the brain is missing)
• defects in the digestive system, such as a problem with the abdomen or esophagus (food pipe)
• certain serous skin conditions.

The blood screening tests do not detect all birth defects. Also, an abnormal test result does not mean that the baby has a birth defect. If a screening test indicates your baby may be at high risk for a birth defect, the test will probably be repeated and other, more specific tests may then be done. Most women with abnormal screening tests have a normal baby.

Who should have this test?

Your health care provider may recommend this test if:

• You will be 35 years old at the time the baby is due.
• You have a family or personal history of birth defects.
• You used certain medicines before or during the pregnancy.
• You have had a particular type of viral infection during your pregnancy.
• You have used illegal drugs or have had a lot of alcohol during the pregnancy.
• You are diabetic and use insulin.
• You have been exposed to a lot of radiation or certain chemicals.

How is the test done?

A small amount of blood is taken from your arm with a needle between the 15th and 18th weeks of the pregnancy. The blood is collected in tubes and sent to a lab.

How will I get the test result?

Ask your health care provider when and how you will get the result of your test.

What does the test result mean?

The test cannot diagnose a birth defect but it can calculate the risk of certain birth defects. The lab calculates the risk based on the levels of the 3 (or more) substances. The age of your unborn baby, as well as your age, weight, race, and whether you have diabetes requiring insulin treatment are included in the calculation because these factors influence the results. Not all tests are 100% accurate and a birth defect may still be present that was not detected by the blood tests. However, if all the tests are normal, your chances of having a baby with a birth defect is very low.

These blood screening tests detect 95% of cases of anencephaly, 80% of cases of serious spina bifida, and 60% or more cases of Down syndrome.

What if my test results are not normal?

Up to 100 of every 1,000 women who have these blood screening tests have abnormal results. An abnormal test result does not mean that a baby has a birth defect. Most women with abnormal results have healthy babies. A common cause of abnormal results is an incorrect due date. Another common cause for abnormal results is a pregnancy with more than 1 baby, such as twins.

If the test results are not normal, your health care provider may repeat the screening tests. Other possible follow-up tests are:

• Ultrasound exam, which helps determine the baby's age and can show if you are carrying more than 1 baby. It can also detect some serious birth defects.
• Amniocentesis, which is a way to get cells from the baby that are in the amniotic fluid. The cells can be studied in the lab to look for chromosome problems, such as Down syndrome. The level of AFP in the fluid may also be tested.
• Cordocentesis, or percutaneous umbilical blood sampling, which is a way to get cells from the baby's blood that can be tested for chromosome problems.

Talk to your health care provider about your results and ask questions. Ask if and when you need more tests.