Chorionic Villus Sampling (CVS)
Chorionic Villus Sampling (CVS)
Chorionic villus sampling (CVS) is a relatively new form of a prenatal testing, completed earlier in gestation than the more traditional testing method, amniocentesis.
Through CVS, small samples of the trophoblast are obtained for chromosome or DNA analysis. The use of CVS as a tool for fetal karyotyping at 10 weeks’ gestation was introduced in 1969, but it did not become accepted practice until later, with the introduction of an ultrasound guided technique for aspiration of material. A larger amount of DNA is derived from CVS than from cells obtained at amniocentesis, thereby allowing more reliable DNA and biochemical analysis in a shorter period of time. A transvaginal sonographic examination usually precedes the procedure to determine the number of embryos and chorionicity (number and type of placentas) and to determine fetal viability.
CVS can be performed in two ways. In transcervical testing, after a speculum insertion into the vagina, a catheter is gently passed through the cervix into the uterus. Guided by ultrasound, the catheter is inserted parallel to the placenta. The stylet (surgical probe) is then removed and a syringe attached; this creates a negative pressure that produces a tissue sample from within the placenta called chorionic villi, which contain the same genetic material as the fetus.
These same fibers can also be extracted by trans-abdominal CVS, when a thin needle guided by ultra-sound real-time monitoring is inserted through the abdomen into the uterus. Again, ultrasound helps identify the exact location of the placenta.
The advantage of CVS is that it can be completed as early as 10 weeks into the pregnancy (but is preferably done closer to 12 or 13 weeks). Villi are processed for cytogenetic analysis in two ways: Cytogenetic results are usually available within 48 hours in the so-called “direct preparation” that is performed on the trophoblastic cells. “Culture method” results are usually available within one week and are performed on the mesenchymal cells. Several laboratories wait in order to report a single result at the same time.
In multiple gestations, uncertain results requiring further investigation are more frequently encountered in CVS than by amniocentesis. Mesenchymal cells are more related to the embryo than trophoblastic cells, because they are derived from the more recently separated tissue that envelops the embryo at this stage of pregnancy, the extraembryonic mesoderm. In some cases, CVS results in confined placental mosaicism— defined as a discrepancy between the chromosomes of placental and fetal tissues. An abnormal karyotype in the mesenchymal cells, instead, is more likely to be associated to an abnormal fetal karyotype.
CVS was initially considered riskier than amniocentesis because it produced a higher chance of miscarriage. With advances in procedures and training since its introduction in 1983, however, CVS is now considered as safe as amniocentesis and is the preferred method for those women who need to obtain early information about the fetus. An excess rate of fetal losses of 0.6-0.8% for the CVS over amniocentesis has been reported.
Maternal complications of CVS, even if not frequent, can occur, although such correlations with CVS are controversial.
Wijnberger, L. D., Y. T. van der Schouw, and G. C. Christiaens. “Learning in Medicine: Chorionic Villus Sampling.” Prenatal Diagnosis 20 (2000):241.