Testing For Genetic Disorders
You and your doctor will first decide if testing for certain birth defects or inherited (genetic) disorders is appropriate for you. These tests help determine if your baby is at higher-than-average risk to have a problem.
Be sure to tell your doctor details of your medical history and family medical history. Your doctor will need this information to help decide if testing is right for your and which tests to consider. Also, your personal beliefs are an important part of the discussion about prenatal testing.
There are two broad categories of prenatal tests — screening tests and diagnostic tests.
Screening tests help to determine if the risk for a particular condition is higher or lower than the risk suggested by your age and medical history. Screening tests do not identify for certain if a pregnancy is affected with a particular condition. But they can help you and your family decide if further testing is appropriate for you.
Doctors may offer a pregnant woman screening tests to help evaluate risk for certain genetic conditions. Most often the tests look for a missing and/or extra chromosome. Chromosomes are pieces of genetic material or DNA. Our cells normally have 46 chromosomes, 23 from the mother and 23 from the father. In some cases there are 45 or 47 chromosomes.
The most common such condition is Down syndrome in which there is an extra copy of chromosome number 21. Affected individuals may have a variety of physical malformations and mental retardation.
First trimester screening. 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. Sometimes other elements seen on ultrasound (such as the appearance of the developing nasal bone) are factored in. 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.)
Second trimester screening. Second trimester ultrasounds are 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 variations of normal that indicated an increase in the risk for conditions like Down syndrome. An increased risk is not necessarily a high risk and most pregnancies with these variations prove to be unaffected.
Depending on the risk you had before having the ultrasound (based on screening tests discussed above, for example) your doctor may ask you to consider further, diagnostic testing such as that discussed in the next section.
Second trimester maternal serum tests. Women who had first trimester serum screening generally will not need second trimester serum tests. However, sometimes blood levels in the second trimester are used to refine risk determined from first trimester screening (so called "sequential" screening).
Second trimester serum tests are also known as the "triple test" and the "quad test." One or the other is offered to pregnant women between the fifteenth and eighteenth weeks of pregnancy. These blood tests -- usually performed on a single sample of blood -- measure the levels of certain substances present in a woman's blood during pregnancy. Three substances are measured in the "triple" test and 4 in the "quad" test. In addition to screening for chromosomal problems, abnormal levels of these substances may indicate the possibility of a neural tube defect (a problem with the skull or spinal cord such as spina bifida or anencephaly).
Diagnostic tests can tell with near certainty if a condition is present or not. To test for genetic conditions affecting a pregnancy, diagnostic tests require a sample of cells from the fetus so that genetic material can be directly analyzed.
In past years diagnostic tests were often considered for women who would 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 complications from the procedure.
Increasingly diagnostic tests for genetic conditions are considered to women of all ages if prior screening tests suggest a higher than average risk of Down syndrome or other genetic abnormality. You and your doctor will discuss the risks vs benefits. Diagnostic tests used for this purpose include:
- Chorionic villus sampling takes a small amount of 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 (approximately 1%).
- 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. 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 carries a small risk of causing miscarriage (less than 1%).
Remember, serum marker measurements and ultrasound exams 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 an increased risk on serum marker screening have no problems at all. An abnormal screening test is a reason to consider diagnostic testing, such as amniocentesis or chorionic villus sampling.