Like amniocentesis, chorionic villus sampling (CVS) is done to obtain a sample of the baby's tissue for determining genetic disorders. In contrast, CVS obtains tissue from the placenta (chorion) instead of fluid. Because the chorion is derived from the baby, not the mother, it bears the genetic characteristics of the baby. The procedure is similar to amniocentesis except that the tissue can be taken by inserting a needle through the abdomen or a sample tube through the cervix.
Like amniocentesis, CVS is done when the mother has one or more risk factors for genetic diseases. CVS can be done earlier than amniocentesis, usually in the latter part of the first trimester ( between weeks nine and 11). The results of CVS can be obtained sooner than amniocentesis because no culturing of the tissue is required. However, because CVS only gets tissue, some of the biochemical tests done with amniocentesis cannot be done with CVS. Also, CVS carries a higher risk (about 1 percent), mostly because it is a newer procedure relative to amniocentesis.