Why Is Chorionic Villus Sampling (CVS) Done?

Chorionic Villus Sampling (CVS) is advised in pregnancies when there is a high risk of the baby having a serious inherited condition. CVS is a prenatal test that helps in detecting specific abnormalities in the unborn baby.

The placenta plays a vital role in providing nutrients and oxygen to the unborn baby, while also removing waste products from the baby's blood. The wispy projections around the placental tissue, which share the baby's genetic constitution, are known as 'chorionic villi'.

For the test, a sample of chorionic villi (sample of cells) from the placenta is removed to test for any genetic defects. The sample is taken through the cervix or the abdominal wall. CVS is also known as 'Chorionic Villus Biopsy'.

How Early Can A CVS Test Be Done?

Generally, CVS is done between 10 and 13 weeks of pregnancy, much earlier than other prenatal diagnostic tests, such as amniocentesis. It can reveal whether a baby has a chromosomal condition or other genetic conditions such as Down's syndrome or cystic fibrosis.

Why Is CVS Done?

A CVS can provide valuable information about your baby's health. Not everyone who is pregnant will have to do a CVS. Usually, CVS is done when the test results can leave a major impact on the management of the pregnancy or on your desire to continue your pregnancy.

You may consider CVS under the following conditions:

• If the results of a prenatal screening test, such as the first-trimester screening turns out to be worrisome. In such a case you can go in for CVS to confirm or rule out the diagnosis.

• In case you had a baby affected by Down's syndrome or any other chromosomal condition in your previous pregnancy, then you might have to consider CVS.

• If you are 35 years of age or older and have a higher risk of chromosomal conditions, you may be required to undergo CVS.

• In case you or your partner have a history of a specific genetic condition or are a known carrier of a genetic condition, then in addition to identifying Down's syndrome, CVS may be used to diagnose other genetic conditions, including single genes disorders like cystic fibrosis or Tay-Sachs.

• In case you have had spontaneous abortions too, your doctor may consider a CVS.

The test has the ability to detect more than 200 types of genetic and biochemical conditions. Therefore, getting the test done during early pregnancy gives parents the necessary information when they have to cope with some major complications of pregnancy, or when they face the possibility of terminating the pregnancy.

Is CVS Painful?

Although CVS is not a very painful procedure, you may experience bleeding or cramping, if the procedure was performed through the cervix rather than the abdomen.

What Are The Risks Associated With CVS?

Infection

Just as is the case with any invasive procedure, there is a risk of infection, although this complication can occur rarely.

Rh sensitization

During the procedure, there is a possibility of the baby's blood getting mixed with the mother's blood. If you have Rh-negative blood, and if your baby's blood is Rh-positive, you may get sensitized. This may result in your body producing antibodies that attack your baby's blood cells. In such a case, your doctor may prescribe medication to prevent you from becoming sensitized. Therefore, if you are Rh-negative, it is essential to keep your doctor informed.

Miscarriage

Although the risk of miscarriage due to CVS is minimal, with chances being less than 1 in 100, the risk for miscarriage increases if the procedure is performed through the cervix instead of the abdomen. The risk is high if the size of the foetus is small for the gestational age.

Foetal deformities

In rare cases, CVS is said to have caused deformities to the baby's limbs, toes and fingers. But the risk is low when the procedure is done after nine weeks of pregnancy.

How Is CVS Different From Amniocentesis?

CVS is an alternative to amniocentesis. In amniocentesis, a sample of the mother's amniotic fluid is taken for testing, whereas in CVS, chorionic villi from the placenta is taken for testing.

CVS can be carried out between 10 and 13 weeks of pregnancy, while amniocentesis is usually done between 15 and 20 weeks of pregnancy.

It takes two to three weeks for the results from amniocentesis to come through, which implies that your pregnancy may reach a more advanced stage before you consider the results. But in CVS, the first results are obtained within the first few days.

Results Of CVS

Once CVS is done, a sample of the CV would be taken to a laboratory, so that the cells are examined under a microscope. The number of chromosomes in the cells is counted and the structure of the chromosomes is checked for abnormalities. If the CVS is being carried out to test for a specific genetic disorder, the cells in the sample can also be tested for this. The test results of CVS are considered to be 99% accurate.

However, it has to be agreed that it cannot test for every birth defect, and the results may not be conclusive. In rare 1% of cases, CVS results may not be able to establish for sure that the chromosomes of the foetus are normal. In such a case, other tests like examining chromosomes in parental cells and other tests like amniocentesis may be necessary to confirm the diagnosis. The first result of CVS would be out in a few days, and you can know if there is a major chromosome problem.