The chorionic villi are within the trophoblastic layer that surrounds the gestational sac early in development, forming the placenta. Chorionic villi have a full set of fetal chromosomes. CVS is a biopsy procedure, removing a tissue sample that contains dividing cells whose chromosome patterns can be evaluated directly. These cells can be cultured and karyotyped. The villi are part of the developing placenta, outside the embryo and outside the surrounding membranes. |
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Seems like a great
idea: sample tissue OUTSIDE the gestational sac and get fast, early
information. But there
are some serious problems. When CVS was introduced, sampling was with a firm
catheter that has to be passed through a closed cervix. Catheter passage
and
aspiration of villi may hurt, can disrupt attachment of the sac, and can
be complicated by bleeding and infection. The catheter method was replaced
by
the use of a needle (like an amnio), however, in order to get enough intact
villi, a LARGE needle with a cutting tip is necessary.
Another problem of CVS is that disrupted villi can be introduced into
the circulation of the embryo, potentially stopping blood flow to
parts of the body. This effect was demonstrated very graphically by DR J. Quintero using video embryoscopy during CVS sampling. |
Some facilities with a lot of experience in CVS have been able to decrease procedure risks significantly, as we have strived to do ourselves for early amniocentesis. Initially, CVS was perfrmed as early as 8 weeks GA. Facilities continuing to do CVS now tend to aim for 12 weeks, which is right in the early amnio range. What is most important is not which procedure is done, but that each patient and her doctor work together to do what they both decide best and safest for the pregnancy in their regional vicinity.
