Detecting Genetic Abnormalities

Some tests can detect genetic abnormalities before birth. By learning about these problems before birth, you can help plan your child’s health care in advance, and in some cases even treat the disorder while the baby is still in the womb.

• With amniocentesis, the doctor inserts a thin needle through the pregnant woman’s abdominal wall and into the uterus, where a small sample of amniotic fluid is withdrawn from the sac surrounding the fetus. When the fluid is analyzed in the laboratory, it can indicate (or rule out) serious genetic and chromosomal disorders, including Down syndrome and spina bifida. Amniocentesis is usually performed during the second trimester (between the fifteenth and twentieth weeks of pregnancy), although it may be done later (typically after the thirty-sixth week) to test whether the baby’s lungs are developed enough for birth. Results of most amniocentesis tests are available within about two weeks.
• With chorionic villus sampling (CVS), a long, slender needle is inserted through the abdomen to remove a small sample of cells (called chorionic villi) from the placenta. Or a catheter (a thin plastic tube) is placed into the vagina and then inserted through the cervix to withdraw cells from the placenta. This sample is then analyzed in the laboratory. CVS is usually performed earlier during the pregnancy than amniocentesis, most often between the tenth and twelfth week of pregnancy, and the test results are available within one to two weeks. It can be used to detect various genetic and chromosomal conditions, including Down syndrome, Tay-Sachs disease, and (especially in African American families) so-called “hemoglobinopathies” such as sickle cell disease and thalassemia.

Both amniocentesis and CVS are considered accurate and safe procedures for prenatal diagnosis, although they pose a small risk of miscarriage and other complications. You should discuss both the benefits and the risks with your doctor and, in some cases, with a genetic counselor.