Pregnancy: Chorionic Villus Sampling

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Chorionic villus sampling (CVS) is a prenatal test that is used to detect birth defects, genetic diseases, and other problems. During the test, a small sample of cells (called chorionic villi) is taken from the placenta where it attaches to the wall of the uterus.

Chorionic villi are tiny parts of the placenta that are formed from the fertilized egg, so they have the same genes as the baby.

If you have certain risk factors for having a baby with a birth defect or genetic disease, you may be offered CVS to detect the problem early in pregnancy.

What diseases or disorders can CVS identify?

CVS can help identify chromosomal problems such as Down syndrome or other genetic diseases such as cystic fibrosis, Tay-Sachs disease and sickle cell anemia. CVS is considered to be 98% accurate in the diagnosis of chromosomal defects, such as Down syndrome.

Since the procedure also identifies the sex of the fetus, it can also help identify disorders that are linked to one sex (such as certain types of muscular dystrophy which occur most often in males).

CVS does not detect open neural tube defects such as spina bifida. If you have CVS, you may want to also consider having a blood AFP test later in pregnancy to screen for those problems.

What are the benefits of CVS?

CVS can be done early in pregnancy (earlier than amniocentesis), and results are usually obtained within 10 days. Getting this kind of information early allows a woman to make choices in the beginning stage of her pregnancy. If a woman chooses to terminate the pregnancy after receiving abnormal test results, the pregnancy termination will be safer than if she waits for amniocentesis results.

What are the risks of CVS?

CVS may carry a slightly higher risk of miscarriage than amniocentesis, since the procedure is done in early pregnancy. Infection may also occur. Rare cases of defects in baby's fingers or toes have been reported, especially when CVS was done before 9 weeks. This risk seems unrelated to the experience of the doctor performing the test or how the test was done. Due to this potential risk, 10 weeks is generally the earliest recommended time to perform this test.

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