Chorionic villus sampling (CVS) is a process of testing the fetus during pregnancy for congenital abnormalities. A piece of the placenta is taken for biopsy early during pregnancy. The placenta has the same genetic and biochemical make up as the fetus. The test is usually done between the 10th and 12th week of pregnancy and is as safe as amniocentesis. It scores over amniocentesis in that it can be done earlier in pregnancy.
When is it done?
The test is recommended in women who are at risk of having a complicated pregnancy. Some of the cases in which the test is recommended are:
Mothers older than 35 years
A prior child with genetic abnormality
Family history of children with chromosomal abnormality
The mother has had two or more miscarriages
Family history of a single sex disorder like muscular degeneration, haemophilia etc.
Which conditions can be detected by CVS?
Some of the common conditions that can be diagnosed by the test are:
Sickle cell anaemia
How is it done?
Before the procedure is performed, an ultrasound examination of the pelvis is done to ascertain the position of the placenta and the size and position of the fetus. This determines which kind of CVS to be done. The procedure can be done by two ways – through the abdomen (transabdominal) and through the vagina (transvaginal or transcervical).
The patient is made to lie on her back with her feet up in stirrups. In the transvaginal procedure, the area around the vagina is cleaned and a thin, plastic tube is inserted through the vagina and guided through the cervix to reach the placenta to suck out a small amount of the tissue. The access is monitored on an ultrasound machine. In the transabdominal procedure, a local anaesthetic is given to numb the area. Then a needle is inserted into the placenta through the abdomen and a small amount of the tissue is sucked out.
Both the procedures are relatively pain free. A slight cramping may be felt in the transvaginal procedure. A little abdominal heaviness may be felt after the anaesthetic wears off.
What happens after the procedure?
The patient is allowed to go home after a couple of hours. Strenuous activity should be avoided for 1-2 days. There may be a little spotting for about a week, which stops automatically. This is more so if the transvaginal procedure is followed. Preliminary results are available within 4 days while the final result may take about 2 weeks.
What is the advantage of CVS?
The biggest advantage of CVS is that it can help detect genetic or chromosomal abnormalities very early in pregnancy. This leaves enough time for the termination of pregnancy, if required.
What are the disadvantages or risks of CVS?
In some cases, albeit rare, the test may be misleading. The placenta may sometimes have a genetic defect that may not harm the fetus itself. In such cases, further testing is required.
The biggest risk of the procedure is miscarriage. About 1-2 pregnancies in a 100 result in miscarriage. This rate is a little higher than that of amniocentesis (1/200 pregnancies). Some other risks may be:
Rupture of the fetal membranes
Leaking of amniotic fluid
Cramping and vaginal bleeding
In rare cases, infection resulting in flu-like symptoms
The procedure should not be done in women who are in an advanced stage of their pregnancy. It should also not be done if the shape of the uterus is unusual or if there is vaginal infection. A disadvantage of the test is that it does not detect neural defects like spina bifida etc.