Chorionic villus sampling – CVS
Chorionic villus sampling (also known as CVS) is an invasive prenatal test performed 10-13 weeks into the pregnancy to detect genetic diseases.
CVS is the earliest prenatal diagnostic test that can eliminate or diagnose genetic diseases and defects in the fetus. It is usually not routinely performed and is offered when there is a significant risk of genetic disorders.
During CVS, a thin needle is used to extract a sample of chorionic villi from the placenta, while an ultrasound helps verify the fetus’ position in the uterus. The sample is sent to a lab and tested for genetic disorders. The procedures performed at the lab include karyotyping, FISH tests and other tests for specific diseases according to risk factors and genetic specialists’ recommendations.
Since the procedure involves inserting a needle into the placenta, it carries a higher risk than amniocentesis. CVS may cause certain complications, including a risk for miscarriage. The risk for miscarriage after chorionic villus sampling is around 1-2%, which is double than the risk after amniocentesis.
However, in cases of a significant risk of genetic disorders, doctors will usually not recommend waiting for amniocentesis that is performed during week 16 of pregnancy and recommend undergoing CVS during earlier stages of pregnancy.
When is CVS performed?
Chorionic villus sampling can be performed earlier than other diagnostic tests, which makes it the test of choice in cases of a significant risk for genetic disorders.
The test is recommended when one of the parents has a genetic disorder with a dominant inheritance pattern, or in cases on balanced translocations when the chances of passing down a defected gene are 50%.
CVS Is also recommended when both parents are carriers of a certain genetic disorder (carry one defected gene and one healthy gene). In these cases, the chances of the baby developing the disorder is around 25%. Such disorders include Tay-Sachs disease.
When there are significant concerns that the fetus has a genetic disorder, an early diagnosis is important to allow parents to choose to terminate the pregnancy. The longer the wait, the more complicated curettage (D&C) becomes, and the chance of complications increases.
CVS test results are available within a month. This makes it highly important to undergo this test as early as possible.
Differences between CVS and amniocentesis
CVS, which is performed in week 10-13, allows terminating the pregnancy up to weeks 14-17. Amniocentesis is performed in a more advanced stage of pregnancy and therefore allows pregnancy termination at a later stage, around 20-24 weeks into pregnancy.
Choosing to wait for amniocentesis results in a later termination of the pregnancy, when the fetus is physically larger. This requires a more complicated procedure that carries a higher risk. In addition, since fetal movement can already be felt at these advanced stages, the procedure may be more complicated emotionally.
Since CVS is performed early into the pregnancy, certain complications may occur. These include:
Increases risk of miscarriage: in around 1-2% of procedures, CVS may lead to miscarriage.
Limb defects: in rare occasions, CVS has been associated with limb defects in the fetus. This risk increases the earlier the test is performed.
Inadequate sampling: since CVS is a relatively complicated procedure, certain problems in cell sampling may arise. This can make the sample unsuitable for testing. In such cases, waiting for amniocentesis is inevitable. Parents may also choose to undergo other genetic tests instead.