Chorionic villi are small structures in the placenta that act like blood vessels. These structures contain cells from the developing fetus. A test that removes a sample of these cells through a needle is called chorionic villus sampling (CVS).
CVS answers many of the same questions as amniocentesis about diseases that the baby might have. Diseases that can be diagnosed with CVS include Tay-Sachs, sickle cell anemia, cystic fibrosis, thalassemia, and Down syndrome. (Rh incompatibility and neural tube defects, however, can be diagnosed only through amniocentesis.) CVS can be done earlier in pregnancy than amniocentesis and can be done when there is not enough amniotic fluid to allow amniocentesis. However, it has some extra risks when compared with amniocentesis.
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