Screening for Birth Defects
Screening tests can give information about a pregnant woman's risk of having a baby with certain birth defects or genetic conditions. These tests also can help your doctor detect possible problems during your pregnancy. Some pregnant women may have other tests, depending on their medical histories, previous pregnancies, family or ethnic background, or exam results. This pamphlet will tell you more about:
Almost all children in the United States are born healthy. Out of 100 newborns, only two or three have major birth defects. A birth defect is a physical problem that is present at birth. It also is called a congenital disorder or malformation. For about 70% of babies born with birth defects, the cause is not known. In other cases, birth defects are inherited through genes or chromosomes (see box) or caused by the mother being exposed to harmful agents or medications.
Birth defects are caused by an error in the way the baby develops. A birth defect may affect how the body looks, works, or both. Many birth defects are mild, but some can be severe. Babies with birth defects may need surgery or medical treatment. Most birth defects occur during the first 3 months of pregnancy. Some can be found before birth with special screening tests. Others appear at birth or later in a person's life. Some of the most common birth defects found through screening tests include:
Some genetic disorders are more common in certain ethnic groups. Carrier testing can be done before, after, or during pregnancy to assess the risk of some of these disorders (see box). This type of testing is not available for all genetic disorders.
Who Should Be Tested?
Screening tests are offered to all pregnant women to assess their risk of having a baby with a birth defect or genetic disorder. If a screening test shows an increased risk of having an affected baby, further tests may be used to diagnose the problem. An abnormal screening test result, while alarming, only signals a possible problem. In most cases, the baby is healthy even if there is an abnormal screening test result. Likewise, a birth defect can occur even if the test result does not show a problem. Most tests focus on a certain problem, and not all disorders can be found by testing.
Women at increased risk of having a baby with a birth defect may be offered a diagnostic test first rather than having a screening test. These risk factors may include:
Types of Screening Tests
Screening tests are easy to perform and do not pose any risks for the fetus. A variety of tests are available that can be done based on the stage or trimester of your pregnancy. Women often have a choice of having a single test or a combination of tests. Some of these tests are new and are not available everywhere.
Your doctor will explain the risks and benefits of the screening tests to help you make the best choice. If there is a history of birth defects in your family, your doctor may recommend you visit a genetic counselor for more detailed information about your risks.
First Trimester Screening
First trimester screening tests include blood tests and an ultrasound exam. This screening can be done as a single combined test or as part of a step-by-step process. Some women may not need further testing. First trimester screening is done between 11 and 14 weeks of pregnancy to detect the risk of Down syndrome and trisomy 18. The blood tests measure the level of two substances in the mother's blood:
The results of the nuchal translucency screening are then combined with those of the blood tests, and the mother's age to assess the risk for the fetus. In the first trimester this combined test detects Down syndrome in most but not all cases (82–87%). When the nuchal translucency thickness is increased, the fetus may have a heart defect or other genetic condition. In this case, your doctor may suggest a more detailed ultrasound exam around 20 weeks of pregnancy.
Second Trimester Screening
In the second trimester, a test called "multiple marker screening" is offered to screen for Down syndrome, trisomy 18, and neural tube defects. This test measures the level of three or four of the following substances in your blood:
First and Second Trimester Screening
The results from both first and second trimester tests can be used together to increase their ability to detect Down syndrome. When both the first- and second-trimester tests are used, about 90–95% of the Down syndrome cases can be detected. With this type of testing, the final result may not be available until all tests are completed.
The Next Steps
If the results of a screening test or other factors raise concerns about your pregnancy, diagnostic tests can be done to provide more information. These tests include:
Finally. . .
Screening tests can help assess the risk of a birth defect. Some tests are offered to all pregnant women. Other tests may be offered based on your history or risk factors. Talk to your doctor about which screening test is right for you.
Anencephaly: A type of neural tube defect that occurs when the fetus's head and brain do not develop normally.
Canavan Disease: A rare disorder that causes the brain to degenerate. Death usually occurs before 4 years of age, although some children may survive into their teens and twenties.
Cystic Fibrosis: A life-long illness in infants, children, and young adults that causes problems with digestion and breathing.
Diabetes: A condition in which the levels of sugar in the blood are too high.
Human Chorionic Gonadotropin (hCG): A hormone produced during pregnancy; its detection is the basis for most pregnancy tests.
Nuchal Translucency Screening: A special ultrasound test of the fetus to screen for the risk of Down syndrome and other birth defects.
Placenta: Tissue that provides nourishment to and takes away waste from the fetus.
Sickle Cell Anemia: A blood disorder in which the red blood cells have a crescent, or "sickle," shape rather than the normal doughnut shape. These cells get caught in the blood vessels, which prevents oxygen from reaching organs and tissues and causes pain.
Tay–Sachs Disease: A disease in which harmful amounts of a fatty substance called ganglioside GM2 collect in the nerve cells in the brain. Tay–Sachs disease causes severe mental retardation, blindness, and seizures.
Trimester: Any of the three 3-month periods into which pregnancy is divided.
Ultrasound Exam: A test in which sound waves are used to examine internal structures. During pregnancy, it can be used to examine the fetus.