Birth Defects Testing
What Are Birth Defects Tests?
Birth defects tests are done during pregnancy to look for possible problems with the baby (fetus). Birth defects develop when something is wrong with the genetic material (genes or chromosomes), an organ, or body chemistry. A birth defect may have only a mild impact on a child's life, or it can have a major effect on quality of life or life span.
Birth defects include:
- Genetic disorders, such as Down syndrome and trisomy 18.
- Diseases passed in a family (inherited), such as Tay-Sachs, sickle cell anemia, and cystic fibrosis.
- Structural problems, such as heart defects and neural tube defects, including spina bifida.
Pregnant women and their partners can choose whether to have a test for birth defects. If you choose to have a test, you may want to talk with a genetic counselor. The counselor can talk with you about the reasons to have or not have the test. He or she can also help you find other resources for support and decision-making.
What Are the Tests?
There are two types of birth defects tests: screening tests and diagnostic tests.
- Screening tests are used to see what the chances are that a baby has a certain birth defect. If the test result is "positive," it means that your baby is more likely to have that birth defect, so your doctor may want you to have a diagnostic test to make sure. If the screening test result is "negative," it means that your baby probably doesn't have that birth defect. But it does not guarantee that you will have a normal pregnancy or baby.
- Diagnostic tests are used to find out for sure if a baby has a certain birth defect.
Some birth defects tests can only be done in the first trimester, while others are only done in the second trimester.
First trimester—Screening tests
First-trimester screening tests include:
- Nuchal translucency test. This test uses ultrasound to measure the thickness of the area at the back of the baby's neck. An increase in the thickness can be an early sign of certain birth defects. This test is often done along with blood tests in the late first trimester. It is not available everywhere, because a doctor must have special training to do this test.
- First-trimester blood tests. These tests measure the amounts of two substances in your blood called beta-human chorionic gonadotropin (beta-hCG) and pregnancy-associated plasma protein A (PAPP-A). Beta-hCG is a hormone produced by the placenta, and high levels may be related to certain birth defects. PAPP-A is a protein in the blood, and low levels may be related to certain birth defects. The doctor looks at the levels of these substances—along with your age and other factors—to help estimate the chance that your baby may have certain problems or birth defects.
First-trimester tests can be done around 10 to 13 weeks of pregnancy, depending on the specific test. Many doctors use a number of tests together, based on what is available.
The nuchal translucency test and the first-trimester blood tests are often done together in what is called the first-trimester screening (FTS). Other names for the FTS include the combined first-trimester screening or the combined screening. This screening test is about as accurate as the maternal serum quad screen, which is done later in pregnancy.1
First trimester—Diagnostic tests
- Chorionic villus sampling (CVS) . Doctors can use this test to look at cells in the placenta. CVS can be done between 10 and 12 weeks of pregnancy. A doctor collects a sample of chorionic villus (placental) cells by putting a thin flexible tube (catheter) into your uterus through your vagina or by putting a needle through your belly into your uterus. The test can be used to find chromosomal birth defects such as Down syndrome and family diseases such as sickle cell disease or cystic fibrosis. But it cannot find neural tube defects.
When CVS is done by a highly trained provider, the chance of it causing a miscarriage can be as low as 1 in 400. This is based on one study. Other studies have shown higher risks of miscarriage after CVS.2
Second trimester—Screening tests
Second-trimester screening tests include:
- Triple or quadruple (quad) blood tests. These blood tests check the amounts of three or four substances in a pregnant woman's blood. The triple screen checks the levels of alpha-fetoprotein (AFP), beta human chorionic gonadotropin (beta-hCG), and a type of estrogen (unconjugated estriol, or uE3). The quad screen checks those three substances, plus the level of the hormone inhibin A. The doctor looks at the levels of these substances—along with your age and other factors—to help estimate the chance that your baby may have certain problems or birth defects.
- Ultrasound. This test allows your doctor to see an image of your developing baby. It is often done at 18 to 20 weeks of pregnancy. Doctors can use ultrasound to screen for certain features that are related to some chromosome problems, such as Down syndrome. Ultrasound also can be used to help find structural problems of the heart, spine, abdomen, or other areas of the body.
Second-trimester tests can be done between 15 and 20 weeks of pregnancy. The triple and quad blood tests have also been called the expanded AFP test, the AFP plus test, or the multiple marker screening test. Some women have first-trimester plus second-trimester screening tests done; these tests are known as integrated screening tests. You will get the results after the tests in the second trimester are done.
Second trimester—Diagnostic tests
- Amniocentesis . This test looks for many chromosomal problems by looking at cells in the amniotic fluid around the baby. A doctor puts a needle through your belly and into your uterus to collect amniotic fluid. This test is done between 15 and 20 weeks of pregnancy (usually around week 16). Amniocentesis also can help find neural tube defects, such as spina bifida.
Is a Birth Defect Test Accurate?
No test is 100% accurate. A screening test may be negative even when the baby has a birth defect. This is called a false-negative test result. It also is possible that a test will be positive—meaning the test result is abnormal—but the baby does not have the problem. This is called a false-positive test result.
A screening test will tell you about your chance of having a baby with a birth defect. If the screening test shows an increased chance, your doctor will probably want you to have chorionic villus sampling or amniocentesis to find out for sure if the baby has a problem.
The first-trimester screening (FTS) correctly finds Down syndrome up to 85% of the time. And the integrated screening correctly finds Down syndrome about 95% of the time.1
Chorionic villus sampling or amniocentesis have an accuracy rate of more than 99% for Down syndrome.4
Should I Be Tested?
The decision to have the test is personal. You must think about your chance of passing on a family disease, your age, your need to know about any problems, and what you might do once you have the test results. Your spiritual beliefs and other values also may play a role in your decision.
You may decide to have the tests because:
- You would think about whether to continue the pregnancy if there is a birth defect.
- You have a family history of an illness such as cystic fibrosis, Tay-Sachs disease, or hemophilia. For more information on cystic fibrosis testing, see the topic Cystic Fibrosis Carrier Screening.
- You have insurance that will pay for some or all of the cost of the tests.
- You need to know whether there might be a problem, because not knowing would make you worry.
- You want to learn all you can about caring for and raising a child with a certain birth defect.
- You would want to plan what hospital and doctors to work with to make sure that all of your baby's needs are met.
Why Would I Not Be Tested?
You may decide not to have birth defects tests because:
- You would continue the pregnancy no matter what the tests show.
- The screening test may be positive even though your baby does not have a problem. You would be worried while you wait for the test results.
- These tests cannot find all possible problems.
- You are worried about the risk of miscarriage. Both CVS and amniocentesis have a small risk of miscarriage. In one study of highly trained providers, the risk from amniocentesis and from CVS was about 1 in 400.2 Other studies have shown higher risks, between 2 and 4 in 400.5 This greater risk may be more likely in medical centers with less experienced providers, especially for CVS.
- Tests can be expensive. You may not have insurance, or your insurance may not pay for the tests.
- American College of Obstetricians and Gynecologists (2007). Screening for fetal chromosomal abnormalities. ACOG Practice Bulletin No. 77. Obstetrics and Gynecology, 109(1): 217–227.
- Caughey AB, et al. (2006). Chorionic villus sampling compared with amniocentesis and the difference in the rate of pregnancy loss. Obstetrics and Gynecology, 108(3): 612–616.
- Cunningham FG, et al. (2005). Ultrasonography and doppler. In Williams Obstetrics, 22nd ed., pp. 389–406. New York: McGraw-Hill.
- Cunningham FG, et al. (2005). Prenatal diagnosis and fetal therapy. In Williams Obstetrics, 22nd ed., pp. 313–339. New York: McGraw-Hill.
- Seeds JW (2004). Diagnostic mid trimester amniocentesis: How safe? American Journal of Obstetrics and Gynecology, 191: 608–616.
Other Works Consulted
- American College of Obstetricians and Gynecologists (2007). Invasive prenatal testing for aneuploidy. ACOG Practice Bulletin No. 88. Obstetrics and Gynecology, 110(6): 1459–1467.
|Author||Sandy Jocoy, RN|
|Editor||Kathleen M. Ariss, MS|
|Associate Editor||Tracy Landauer|
|Associate Editor||Pat Truman, MATC|
|Primary Medical Reviewer||Sarah Marshall, MD - Family Medicine|
|Specialist Medical Reviewer||Siobhan M. Dolan, MD, MPH - Reproductive Genetics|
|Last Updated||May 20, 2008|
Last Updated: May 20, 2008