Prenatal Diagnosis Center
What are Diagnostic Tests and What Types Are Available?
Diagnostic tests identify chromosomal problems such as Down syndrome. We can test for other genetic diseases like Tay-Sachs, cystic fibrosis, sickle cell anemia and thalassemia if the fetus is determined to be at risk. Because cells are tested directly from the fetus or placenta, the results are diagnostic (greater than 99% accurate for chromosome results) and specific for the fetus. However, because it is necessary to enter the womb with a needle or catheter to obtain the cells, there is a small risk of miscarriage due to the procedure. The most recent studies show the risk for miscarriage due to these procedures is 1 in 1600.
Chorionic Villus Sampling (CVS)
CVS is a procedure in which a small amount of tissue (chorionic villi) is obtained from the developing placenta between 10 and 13 weeks of pregnancy. The tissue is then analyzed for chromosome studies and, if indicated, specific genetic diseases. The advantage of CVS is that the test is performed much earlier in pregnancy than amniocentesis. A disadvantage is that CVS does not detect neural tube defects (e.g. spina bifida). However, an AFP test and a high-resolution ultrasound examination can be performed later in pregnancy to screen for these defects. In addition, approximately one percent of CVS results will be unclear or indeterminate. Most of the fetuses in these pregnancies are normal, but additional testing, including amniocentesis, may be indicated.
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Amniocentesis is traditionally performed between 16 and 20 weeks of pregnancy. During the procedure, a thin needle is inserted through the mother’s abdominal wall into the amniotic fluid surrounding the fetus. A small amount of fluid is removed and analyzed for chromosome abnormalities, spina bifida and, if indicated, specified genetic diseases.
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