Testing For Genetic Disorders
A variety of routine prenatal tests can help to determine your chance of having a pregnancy affected by certain birth defects and/or inherited (genetic) disorders. Your doctor may recommend other tests based on your medical history or results from other tests. Discuss the benefits and risks of these tests with your doctor to determine which tests may be appropriate for you. There are two broad categories of prenatal tests:
Screening Tests
Screening tests try to identify babies that might have Down syndrome, a condition that causes physical malformations and mental retardation. These tests cannot tell for sure. They can only tell if your baby is at increased risk of Down syndrome. Tests include:
- Maternal serum screening (also called "triple screen" or "quad screen") is routinely offered to pregnant women between the fifteenth and eighteenth weeks of pregnancy. It consists of blood tests — usually performed on a single sample of blood — that measure levels of certain substances present in a woman's blood during pregnancy. Abnormal levels can indicate the possibility of a neural tube defect (a problem with the skull or spinal cord such as spina bifida or anencephaly) or the possibility of chromosomal problems (such as Down syndrome). Chromosomes are pieces of genetic material or DNA, the blueprints from our body. Our cells normally have 46 chromosomes, 23 from our mother and 23 from our father. In Down syndrome, an extra chromosome is present. In Turner syndrome, a chromosome may be missing.
- Second trimester ultrasound (also called "level 2 ultrasound") is routinely offered to women between 16 and 20 weeks of pregnancy. This test looks for abnormalities in the fetal anatomy, such as a heart malformation. Sometimes the ultrasound shows something abnormal, but the baby turns out to be normal. Your doctor might recommend a diagnostic test if the ultrasound is abnormal.
- First trimester screening (early ultrasound and serum screen) measures the thickness of skin at the back of the baby's neck and measures two markers in the mother's blood. A woman can receive an early ultrasound between weeks 11 and 14. Note: because the test requires special ultrasound training, it is not available in all hospitals.
Diagnostic Tests
Diagnostic tests are for women who will be over age 35 at delivery, who received an abnormal result on a screening test, or who are anxious due to a prior pregnancy. Diagnostic tests require a sample of cells from the fetus. Tests include:
- Chorionic villus sampling is used to detect some genetic defects (such as Down syndrome) by sampling tissue from the developing placenta. It is usually performed between 10 and 12 weeks of pregnancy. A tissue sample is obtained by passing a small needle or tube through the mother's cervix or through the mother's abdominal wall into the uterus. This tissue is analyzed for information about the genetic makeup of the fetus. This test may carry a slightly higher risk of miscarriage than amniocentesis.
- Amniocentesis is usually performed between 15 and 20 weeks of pregnancy to detect chromosomal abnormalities or to provide a more specific diagnosis of a neural tube defect. A needle is inserted into the uterus through the abdominal wall to retrieve a sample of the amniotic fluid that surrounds the fetus. Amniotic fluid contains some skin cells from the fetus. Looking at these cells can provide information about the baby by counting the chromosomes and, if indicated, testing them for certain genetic defects. Amniotic fluid can also be tested for a protein called AFP that is present at higher levels if the fetus has a neural tube defect. Amniocentesis is often considered for women who will be age 35 (or older) at the time of delivery, because the risk of Down syndrome at this age is approximately as high as the risk of the amniocentesis itself. Amniocentesis carries a small risk of causing miscarriage, so the test is generally performed only if an increased risk (of age or other factors) suggests that it is medically indicated.
Remember, serum marker screening and ultrasound are screening tests. An abnormal result from a screening test does not necessarily mean there is a problem. In fact, most babies of women with abnormal serum marker screening have no problems at all. An abnormal test result is an indication for you to consider more specific testing, such as amniocentesis or chorionic villus sampling. Amniocentesis and chorionic villus sampling are called diagnostic tests because they can give a more accurate diagnosis than a screening test.
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