Associated Women's Health Specialists  
     
     

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:
  • What screening tests are available
  • Why and when the tests are done
  • What the results mean

Screening tests can help assess the risk of a birth defect. Talk to your doctor about which screening test is right for you.
Birth Defects

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:

  • Neural tube defect: Incomplete closure of the fetal spine that can result in spina bifida or anencephaly.
  • Abdominal wall defects: One type of defect occurs when the muscle and skin that cover the wall of the abdomen are missing and the bowel sticks out through a hole in the abdominal wall (gastroschisis). Another type is when the tissue around the umbilical cord is weak and allows organs to protrude into this area (omphalocele).
  • Heart defect: The chambers or pathways through the heart are not properly developed.
  • Down syndrome: Mental retardation, abnormal features of the face, and medical problems such as heart defects occur as a result of an extra chromosome 21 (trisomy 21).
  • Trisomy 18: There is an extra chromosome 18, which causes severe mental retardation and birth defects and sometimes death.
Genes and Chromosomes

Traits are passed from parent to child through genes and chromosomes. Each cell in your body is made up of genes and chromosomes. Each chromosome carries many genes. Half of a fetus's genes come from the mother. The other half come from the father. Some traits, such as blood type, are determined by single genes. Other traits—including skin color, hair color, and height—are the result of many genes working together. A gene or a genetic disorder is either dominant or recessive. If one gene in a pair is dominant, the trait it carries cancels out the trait carried by the recessive gene. For a recessive trait to appear, the gene that carries it must be inherited from both parents.

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:

  • Family or personal history of birth defects
  • Previous child with a birth defect or genetic condition
  • Use of certain medicines around the time of conception
  • Diabetes before getting pregnant
Carrier Testing

Some birth defects are inherited. Just as a baby gets certain traits like eye color from the parents, certain diseases or disorders can be passed on to the baby.

A carrier is a person who shows no signs of a particular disorder but could pass the gene on to his or her children. Carrier testing is done to see if a couple carries a gene for certain inherited disorders. It can be done to check for a number of genetic disorders, such as cystic fibrosis, sickle cell anemia, Tay–Sachs disease, thalassemia, familial dysautonomia, and Canavan disease.

For this test, a sample of a person's blood or saliva is studied in a lab. If the test result shows the mother is a carrier, the next step is to test the baby's father. If the test result shows that both parents are carriers, a genetic counselor can provide more information about the risk of having a baby with the disorder.

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:

  1. Pregnancy-associated plasma protein-A (PAPP-A)
  2. Human chorionic gonadotropin (hCG)
An ultrasound exam, called nuchal translucency screening, is used to measure the thickness at the back of the neck of the fetus. An increase in this space may be a sign of Down syndrome, trisomy 18, or other chromosomal problems.

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.

Detailed Ultrasound Exam

During pregnancy, many women have a basic ultrasound exam. A detailed ultrasound exam may be done if there is a risk of a birth defect based on family history or an abnormal result from a screening test. This type of exam allows a more extensive view of the baby's organs and features. A detailed ultrasound exam can generally be done after 18 weeks of pregnancy. Even a detailed ultrasound cannot detect all birth defects.

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:

  • Alpha-fetoprotein (AFP)—A substance made by a growing fetus, which is found in amniotic fluid, fetal blood, and, in smaller amounts, in the mother's blood.
  • Estriol—A hormone made by the placenta and the liver of the fetus.
  • Human chorionic gonadotropin—A hormone made by the placenta.
  • Inhibin-A—A hormone produced by the placenta.
The test using the first three of these substances is called a triple screen. When the fourth substance (inhibin-A) is added, the test is called a quad screen. The triple screen test detects Down syndrome in 69% of the cases. The quad screen detects Down syndrome in 81% of the cases. The AFP test detects neural tube defects in 80% of the cases. These tests usually are done around 15–20 weeks of pregnancy. The stage of pregnancy at the time of the test is important because levels of the substances measured change during pregnancy.

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:

  • Detailed ultrasound exam—A type of ultrasound exam that can help explain abnormal results and provide more detailed information about the growth and development of the fetus.
  • Amniocentesis—A procedure in which a small amount of amniotic fluid and cells are withdrawn from the sac surrounding the fetus and tested.
  • Chorionic villus sampling (CVS)—A procedure in which a small sample of cells from the placenta is tested.
Your doctor can help advise you on which tests may be best for you. He or she also can explain what the results mean.

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.

Glossary

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.