CVS is a procedure in which a small amount of tissue (chorionic villi) is obtained from the developing placenta between 10 and 12 weeks of pregnancy. The tissue is analyzed for chromosome disorders and, if indicated, specific genetic diseases.
On the positive side, CVS is performed much earlier in pregnancy than amniocentesis. On the negative side, CVS does not detect neural tube defects. However, an AFP test and a high resolution ultrasound examination can be performed later in pregnancy to screen for these defects.