What is a chorionic villus sampling (CVS)?

Chorionic villus sampling (CVS) is a prenatal test that involves taking a sample of some of the placental tissue. This tissue contains the same genetic material as the fetus and can be tested for chromosomal abnormalities and some other genetic problems. Testing is available for other genetic defects and disorders depending on the family history and availability of laboratory testing at the time of the procedure. In comparison to amniocentesis (another type of prenatal test), CVS does not provide information on neural tube defects such as spina bifida. For this reason, women who undergo CVS also need a follow-up blood test between 16 to 18 weeks of their pregnancy, to screen for neural tube defects.

This procedure is used during the first trimester for the same purposes as an amniocentesis. (Women usually have one or the other, but not both, if such testing is deemed necessary.) It involves taking a sample of the tissue that attaches the amniotic sac (the sac around the fetus) to the wall of the uterus. Like amniocentesis, CVS is typically done only when there are certain risk factors; its primary advantage is that results are available sooner. CVS also carries a slightly increased risk of miscarriage and other complications. 

How is CVS performed?

CVS may be offered to women who are at increased risk for chromosomal abnormalities or have a family history of a genetic defect that is testable from the placental tissue. CVS is usually performed between the 10th and 12th weeks of pregnancy. Although exact methods can vary, the procedure involves inserting a small tube called a catheter through a woman's vagina and into her cervix and usually follows this process. Another method is transabdominal CVS, which involves inserting a needle through the woman's abdomen and into her uterus to sample the placental cells.

• Ultrasound is used to guide the catheter (for transvaginal CVS) or needle (for transabdominal CVS) into place near the placenta.
• Tissue is removed using a syringe on the other end of the catheter.
• Women may feel some cramping during and after the CVS procedure.
• The tissue samples are sent to a genetic laboratory to grow and be analyzed. Results are usually available in about 10 days to two weeks, depending on the laboratory.
• Women with twins or other multiples usually need sampling from each placenta. However, because of the complexity of the procedure, and positioning of the placentas, CVS is not always feasible or successful with multiples.

Some women may not be candidates for CVS or may not obtain results that are 100 percent accurate, and may therefore require a follow-up amniocentesis. In some cases there is an active vaginal infection such as herpes or gonorrhea, which will prohibit the procedure. Other times the physician obtains a sample that does not have enough tissue to grow in the laboratory, such that results are incomplete or inconclusive.

Read more:

What is an amniocentesis