Chorionic villus sampling (CVS) is a relatively new form of prenatal testing, completed earlier in gestation than the more traditional testing method, amniocentesis. Through CVS, the fetus's genes can be examined and common chromosomal disorders and hereditary conditions, as well as the baby's sex, can be identified.

CVS can be performed in two ways. In transcervical testing, a thin tube is inserted through the cervix and into the uterus. Guided by ultrasound, the tube suctions microscopic projections called chorionic villi from within the placenta, which contain the same genetic material as does the fetus. These same fibers are extracted by transabdominal CVS, when a thin needle is inserted through the abdomen into the uterus. Again, ultrasound assists in identifying the exact location of the placenta.

The advantage of the CVS procedure is that it can be completed as early as 10 weeks in the pregnancy, but preferably closer to 12 or 13 weeks, and results are usually available within 48 hours. Amniocentesis is not done until the 15th to 19th week, and results can take up to two weeks. CVS was initially considered riskier because of increased chance of miscarriage or limb defects. With advances in procedure and training since its introduction in 1983, however, CVS is now considered as safe as amniocentesis and the preferred method for those women who need to obtain early information about the fetus.