Chorionic callus sampling is essentially a placental biopsy, a procedure by which a small amount of placental tissue is obtained. The placenta, as well as the fetus, originate from the same source, the fertilized egg and they share the same genetic material. In that respect genetic tests in the placental cells are diagnostic for the fetus.
The main advantage of the CVS is that it can be performed early in the pregnancy (from the 11th week onwards) offering the benefit of an early diagnosis. The risk of complications does not differ between amniocentesis and CVS.
Rarely (in less than 1% of cases), a chromosomal abnormality may be found in some of the placental cells that may be confined in the placenta and not affecting the fetus (placental confined mosaicism). In those cases amniocentesis is recommended for clarification.