Chorionic villus sampling (CVS) is about 99% accurate for detecting chromosome disorders in the developing baby.
CVS works by taking a tissue sample from the placenta and analyzing the chromosomal makeup, which in most cases will be the same as the baby's. It can confirm or rule out the presence of disorders caused by an abnormal number of chromosomes, such as trisomies. Normal results on a CVS does not guarantee that the baby will not have any genetic disorders, however. It is not currently feasible to scan for every known gene-based disorder during a CVS, although it may be possible to check for specific genetic disorders if there is one that is known to run in your family. CVS also does not provide information about neural tube defects like anencephaly.
About 1% of the time, a CVS may return inconclusive results that require further followup. An example would be CVS results showing trisomy 16. Trisomy 16 almost always causes a miscarriage to occur before the point that a CVS could be performed, so there is a chance that if the pregnancy is still appearing viable at the time of the CVS that the condition might be confined to the placenta or there might be some level of mosaicism. In these types of cases, the doctor may recommend following up with an amniocentesis to get more information.
Sources:
Chorionic Villus Sampling (CVS). Mt. Sinai Hospital. Accessed: Apr. 1, 2009. http://www.mountsinai.on.ca/care/pdmg/tests/cvs/?searchterm=CVS.
Hahnemann JM, Vejerslev LO."Accuracy of cytogenetic findings on chorionic villus sampling (CVS)--diagnostic consequences of CVS mosaicism and non-mosaic discrepancy in centres contributing to EUCROMIC 1986-1992.." Prenat Diagn. 1997 Sep;17(9):801-20.
