Genetic Testing

There are many options for genetic testing. Carrier testing can be done before pregnancy to determine whether a person is a carrier for a particular genetic disorder. Screening tests can be performed during pregnancy to assess the risk that the baby has a certain birth defect. Diagnostic tests are done to determine whether the baby has a particular disorder. At your first prenatal visit, your doctor will discuss the various choices with you and help you choose the testing options that are most appropriate for your specific situation.

Carrier Testing

Carrier testing is a way to find out whether a person is a carrier of a particular disorder. A carrier is a person who shows no signs of a disorder, but could pass the gene on to his or her children. Carrier testing is available for many, but not all, genetic disorders, including cystic fibrosis, sickle cell disease and other blood disorders, Tay–Sachs disease and Canavan disease. Cystic fibrosis carrier testing, for example, is available to all pregnant women and should be offered when both partners are of Caucasian, European or Ashkenazi Jewish ethnicity. Other carrier tests are recommended for people who have certain ethnic backgrounds or who have a strong family history of a genetic disease.

Carrier testing is typically performed before pregnancy to assess the risks of having a child with a particular disorder. If the test shows you are a carrier, the next step is to test the baby’s father. If the test shows that both parents are carriers, a genetic counselor can give you more information about the risk of having a baby with the disorder. Further testing may be available to show if the baby has the disorder or is a carrier.

Screening Tests for Birth Defects

Screening tests are available for certain types of common birth defects. Some of the most common birth defects found through screening tests include neural tube defects, abdominal wall defects, heart defects, Down syndrome and trisomy 18.

Certain screening tests are offered to all pregnant women to assess their risks of having a baby with a birth defect or genetic disorder. If a screening test shows an increased risk of having an affected baby, diagnostic tests may be offered to diagnose the problem. Some women may choose to have diagnostic testing instead of screening testing. However, some diagnostic tests involve some risks. It is important to understand these risks before having these tests.

Screening tests can be performed in the first or second trimesters:

• First-trimester screening: First-trimester screening includes a blood test that measures the levels of two proteins in your blood and a special ultrasound called a nuchal translucency screening. Together, these two tests are known as first-trimester combined screening and are done between weeks 10 and 14.
• Second-trimester screening: Second-trimester screening involves a blood test that is done between 15 and 20 weeks of pregnancy. The test has many different names — the quadruple or quad screen, multiple marker screen and maternal serum screen. The test measures the levels of four substances in your blood, which can tell whether the baby may have a chromosomal problem.
• Combined Screening: The results of first-trimester and second-trimester screening tests can be combined in a number of ways to increase their ability to detect Down syndrome. With this type of testing, the final result may not be available until all the tests are completed. When used together and depending on the tests used, 85 to 96 percent of Down syndrome cases can be detected.

Detailed Ultrasound Exam

A detailed ultrasound exam may be done if there is 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 generally can be done after 18 weeks of pregnancy. Even a detailed ultrasound exam, however, cannot detect all birth defects. Detailed ultrasound testing is best used to detect structural abnormalities, such as abdominal wall defects. Ultrasound carries very little risk for either mother or baby but should be performed by qualified specialists and only when indicated.

Diagnostic Tests for Birth Defects

Diagnostic tests include a detailed ultrasound exam, amniocentesis, chorionic villus sampling and cordocentesis. In the past, women at increased risk of having a baby with a birth defect — for instance, women older than 35 — were offered diagnostic testing instead of screening tests. Now, these tests are offered as a first choice to all pregnant women before 20 weeks of pregnancy. It’s important, however, to understand the risks of these diagnostic tests, which may include an increased risk of pregnancy loss, before choosing this option.

• Amniocentesis: Amniocentesis is performed at 15 to 20 weeks of pregnancy. It can reveal whether a baby has a chromosomal defect (like Down syndrome) and certain other birth defects. To perform the procedure, a doctor guides a thin needle through the abdomen and uterus. A small sample of amniotic fluid is withdrawn and sent to a lab. Complications from amniocentesis include cramping, vaginal bleeding and leakage of amniotic fluid, which occurs in 1 to 2 percent of women having this procedure. There is a slight chance (about 0.5 percent) of miscarriage as a result of amniocentesis (although a recent study found the risk to be as low as 0.06 percent).

• Chorionic villus sampling: CVS detects the same chromosomal problems that amniocentesis does. It can be performed earlier than amniocentesis — often at 10–12 weeks of pregnancy. To do CVS, a doctor guides either a small tube through the vagina and cervix or a thin needle through the abdomen and uterine wall. A small sample of the chorionic villi from the placenta is taken. The cells in the sample are grown in a lab and checked for chromosomal problems and other defects. As with amniocentesis, possible side effects from this procedure are cramping and bleeding. The risk of miscarriage as a result of CVS is about 1 percent. If it performed earlier than 10 weeks, the risks associated with the procedure, including the risk of a limb abnormality, is increased. This risk decreases to almost normal when CVS is performed at 10 weeks of pregnancy.

• Cordocentesis: Cordocentesis (also called fetal blood sampling) is used to test for chromosomal defects and other abnormalities. At 18 weeks of pregnancy or later, blood is taken from a vein in the umbilical cord. This test usually is done when the results of amniocentesis, chorionic villus sampling or ultrasound are unclear. As with amniocentesis, possible side effects from this procedure are cramping and bleeding. The miscarriage rate after cordocentesis is about 1 to 2 percent.