Chorionic Villus Sampling
Chorionic Villous sampling (CVS) is a test usually offered during first trimester of pregnancy to detect chromosomal abnormality or genetic syndrome. CVS test is performed by taking a sample of the chorionic villi (placental tissue). The genetic information in the tissue helps in finding out if an abnormal genetic condition is present like Down syndrome, Thalassemia or other genetic syndrome. This helps them to decide how to proceed with the pregnancy.
Why is this procedure done?
It is most often done to rule out chromosomal and genetic disorders in the fetus. It is usually offered in the following situations:
- When the first trimester screening shows a ‘high risk’ for chromosomal abnormalities
- When an abnormality is detected in the baby on the first trimester ultrasound examniation
- To rule out genetic disorders like sickle cell disease, thalassemia, muscular dystrophy etc.
- When there is family history of a genetic disorder for which the genetic mutation is known
How is it done?
CVS is done by passing a needle through the mother’s abdomen under local anaesthesia to procure a small amount of placental tissue. The test is done under continuous ultrasound guidance so as to avoid the baby. It is usually done between 11 to 13 weeks of pregnancy. The placental tissue is then sent to the laboratory for testing. For some tests, both parents blood samples are also needed.
How painful is the procedure? Will anaesthesia be given to me?
Local anaesthesia is given at the needle puncture site prior to the procedure. Most often the procedure is as uncomfortable or as painful as routine blood sampling.
What are the risks associated with it?
There is small risk of miscarriage of around 1 in 300 for chorionic villus sampling. Although all precautions are taken during the procedure to minimize infection, there also remains a small risk of infection (1 in 1000). In about 1 in 100 patients, the culture may fail and the full karyotype may be unavailable. In some cases, the results may take longer or may not be available because of mosaicism. If a full karyotype is still needed, the doctor will offer you amniocentesis after 15 weeks.
What should I expect before the procedure?
The procedure, its indication and the associated risks will be discussed with you and you will be asked to sign an informed consent. Your blood group and certain other blood tests (HIV, Hepatitis B) will be checked.
What should I expect after the procedure?
You will be asked to rest and the baby’s heart beat will be checked after half an hour. You may have slight bruising at the site of needle entry. You may also have some spotting or mild cramping for a couple of hours. You may take oral paracetamol which is safe in pregnancy. There is no restriction to your daily activities though the doctor may advice you to stay home for 3 days to minimize chances of infection. If you have fever, vaginal bleeding or leaking or contractions, you must report to the hospital.
How long do the results take to come?
The FISH report comes within one week and the full the karyotype report takes 2 to 3 weeks. The timing of other results will depend on the test performed and usually varies between 7 to 10 working days