Birth Defects Testing
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 between 9 and 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.
For more information, see the medical tests Fetal Ultrasound, Alpha-Fetoprotein (AFP) in Blood, Chorionic Villus Sampling (CVS), and Amniocentesis.
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| Author: | Sandy Jocoy, RN | Last Updated: May 20, 2008 |
| Medical Review: | Sarah Marshall, MD - Family Medicine Siobhan M. Dolan, MD, MPH - Reproductive Genetics | |
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