Chorionic Villi Sampling
What is Chorionic Villus Sampling?
Chorionic villi sampling (CVS) is done to determine if your child has a chromosomal disorder such as Tay-Sachs, sickle-cell anemia, most types of cystic fibrosis and Down syndrome. It involves placing a small needle or catheter either through the abdomen or through the vagina near the uterus in order to collect a small sample of placental tissue called chorionic villi. Ultrasound is used at the same time in order to guide the needle or catheter away from the baby and the placenta. The test can be done as early as ten weeks and is most commonly done between the 10th and 13th weeks of pregnancy.
Why Should I Have CVS Done?
CVS is performed in the first trimester, and it can give results or reassurance early in your pregnancy. An early diagnosis is especially helpful for those who may consider a therapeutic pregnancy termination if something is seriously wrong, since an earlier abortion is less complicated and traumatic.
CVS is normally performed in a hospital. The sample of cells from the placenta is taken via the vagina and cervix (transcervical CVS) or via a needle inserted in the abdominal wall (transabdominal CVS). Though this is a fairly common procedure, some discomfort – ranging from mild to severe – may be present during a CVS.
Transcervical CVS involves the insertion of a long thin tube into the vagina. Guided by ultrasound imaging, the doctor directs the tube to the tissue that will eventually become the fetal half of the placenta. A sample is then trimmed off for further study.
With the transabdominal CVS procedure, the doctor will again use ultrasound to determine the exact location of the placenta and to find a safe place where to insert the needle to minimize any trauma to the mother or child. A needle is then inserted through the abdomen and uterus to the edge of the placenta. At this point, a narrower needle is inserted through the guiding needle and a small sample of cells is taken from the chorionic villi for diagnostic study. It is generally recommended that those undergoing the procedure arrange to have someone drive them home afterward and that they make no other plans for the rest of the day since it can be emotionally and physically taxing.
After the samples have been taken, further diagnostic study is performed on them. Since the cells in the sample are actually of fetal origin, they can be used to generate a detailed picture of the genetic makeup of the developing fetus. In most circumstances, the results from the test are available within three to five days.
Risks and Benefits
Most studies so far have indicated that CVS is safe and reliable. However, the procedure is slightly riskier than amniocentesis. An important fact to remember is that there is a very small miscarriage rate associated with this test. There are also some studies that suggest that there may be a slight increase in the number of limb deformities from amniotic banding syndrome. According to most studies, however, inexperience of the technician performing the procedure appears to be the cause of these rare complications.
The primary advantage of chorionic villus sampling is that results are available much earlier in pregnancy. Since it can be performed in the first trimester, CVS can give results earlier in pregnancy than amniocentesis, which is usually performed after the 16th week. Thus, if results are abnormal, simpler and safer methods can be used to terminate the pregnancy, or the fetus can be treated prenatally. Also, normal results decrease parental anxiety earlier. The risks have to be weighed against this benefit of earlier diagnosis and should be discussed at length with your doctor. If a genetic disorder is detected, you will be referred to a genetic counselor for further counseling and decision-making.
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