High-Risk Pregnancy - Chorionic Villus Sampling
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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.
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:
- Ultrasound is used to guide the catheter into place near the placenta.
- Tissue is removed using a syringe on the other end of the catheter.
- Another method is transabdominal CVS, which involves inserting a needle through the woman's abdomen and into her uterus to sample the placental cells.
- 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.
The risks of CVS include the following:
CVS has a similar or slightly higher rate of pregnancy loss compared to amniocentesis. Consult your physician for information about loss rates.
CVS is an invasive procedure and may cause infection.
- limb defects
Some reports show that babies born after CVS may have an increased rate of limb (arm, leg, or finger) defects. Other studies have not shown an increase, except when CVS is performed before 9 weeks of pregnancy.
The benefits of CVS include the following:
- early diagnosis of a chromosomal abnormality
- early diagnosis of certain genetic defects
Be sure to discuss the risks and benefits of CVS more specifically with your physician.