Chorionic villus sampling, CVS, is a procedure that can obtain genetic information about the unborn baby. CVS is typically done between 10-12 weeks. The advantage to CVS over amniocentesis is that it can be done in the first trimester. This is important as it allows termination for genetic abnormalities to be performed at the end of the first trimester. This is a much simpler and safer procedure than second trimester termination. CVS has about a 1% risk of causing a miscarriage, this is about twice the risk of amniocentesis. CVS can have both false positive and false negative results.
Chorionic villus sampling is a biopsy of the placenta. It is performed by either placing a needle through the lower abdomen or through the vagina depending on the location of the placenta. A biopsy of the chorionic villi, a part of the placenta, is done through the needle. This biopsy can then be sent for chromosome studies looking for genetic abnormalities such as down syndrome and trisomy 18. CVS is often paired with first trimester screening. If the first trimester screen is abnormal, CVS may be offered to confirm that the baby has a problem.
CVS is not available in all areas. CVS is a somewhat challenging procedure and requires a skilled physician to keep the risk low. The 1% risk of miscarriage in true for a skilled physician who performs many of these procedures. The risk is much greater if CVS is performed by someone with less experience or skill. Because of this, not every community has a physician that can safely perform CVS. People that live in a large city or near a major medical center are more likely to have CVS available than someone who lives in a smaller community. Ask your obstetrician if CVS is something that is available in your area, this information may effect the decision about whether or not to do first trimester screening.
CVS can give false positive results. This means that it possible for CVS to say the baby has down syndrome or other genetic abnormality when in fact it does not. This risk is low but not zero. False positive CVS can occur when the placenta exhibits mosaicism. This is when some of the placenta exhibits cells with abnormal chromosomes and other parts of the placenta have normal chromosomes. In these cases the baby may be perfectly normal. Because CVS samples the placenta and not the baby, if the abnormal cells of the placenta are the ones that are sampled a false positive result may be obtained. False positives in amniocentesis are almost never seen.
CVS may also give false negative results. This can occur when some of the mothers cells are biopsied instead of the cells truly coming from the placenta. In this case the results will be the mother’s chromosomes but it will appear as if they belong to the baby. If the baby has a genetic abnormality it will be missed. This is unusual but it can happen. False negative results are almost never seen in amniocentesis.
Another disadvantage to CVS is that it does not test for neural tube defects. These are problems with the brain stem and spinal chord, spinal bifida is the most common of this class of abnormality.
The main advantage of CVS over amniocentesis is that CVS can be done at the end of the first trimester while amniocentesis is not done until about 16 weeks. CVS has some disadvantages. It has a greater risk of compilations, namely causing a miscarriage. CVS can have both false positive and false negative results, these are very rare with amniocentesis. CVS does not test for neural tube defects, amniocentesis does. And CVS is not available in all areas.