Chorionic Villus Sampling (CVS)

Chorionic villus sampling or CVS is an earlier alternative to amniocentesis. CVS is a procedure, which involves the withdrawal of a small amount of the developing placenta that is made up of chorionic villi. These villi are finger-like projections that contain cells that are representative of the baby. They can be analyzed to determine the chromosomal status of the baby and in special circumstances, to determine the presence or absence of particular genetic disorders. CVS is typically performed between 10 to 12 weeks gestation, counting from the first day of the last menstrual period, or as determined by ultrasound examination. You must be at least 10 weeks gestation to have a CVS. You can be further along than 12 weeks gestation to have a CVS.

The risk of having a baby with certain chromosomal abnormalities increases, as a woman gets older. The most common of these disorders is Down syndrome. You also have an additional risk of having a child with other types of chromosomal abnormalities. CVS can test for the majority of chromosome problems, such as Down syndrome, associated with a women’s age. Some of these abnormalities are severe in nature, some are mild, and relatively few are unpredictable in their effects on human development.

In order to understand these chromosome problems, it may be helpful to briefly review chromosomes and their function. Chromosomes are the structures in our cells that carry our genetic information or genes. Normally, we have 46 chromosomes in our cells. When we have a child, they inherit half (23) of their chromosomes from the mother, in the egg, and the other half (23) of their chromosomes from their father, in the sperm. The most common chromosome problems, such as Down syndrome, occur when an additional or extra chromosome is present in the cells of the baby. Many of these chromosome problems can be detected by CVS.

Each chromosome carries many genes. Genes have many functions including control of our growth and development, as well as the determination of many traits. Many disorders caused by an error in a single gene can also be detected by CVS for families at risk for these conditions.

Procedure:

There are two approaches to sampling the developing placenta: transcervically or transabdominally. The method that is used is dependent upon the position of the placenta and uterus. This will be determined the day of the procedure during the ultrasound examination. A thorough ultrasound examination is performed prior to and during the procedure.

• For the transcervical CVS, a thin hollow plastic tube or catheter is used to obtain a small piece of the placenta. A speculum is inserted, and the catheter is guided through the vagina and cervix and into the placenta. A small sample is then removed. Many women report this procedure feels similar to a Pap test.
• For the transabdominal CVS, a slender needle is inserted through the abdomen into the uterus and placenta. A small piece of the placenta will be removed by applying gentle suction to the syringe attached to the needle.

In our experience, either way the procedure is performed, it typically requires only a few minutes. Including the ultrasound prior to the procedure, the entire process typically requires approximately 20 minutes. Most women report minimal discomfort associated with both procedures.

Results:

In order to obtain the test results, the chromosomes within the placenta’s cells are studied under a microscope. The number and general structure of the chromosomes are checked. Also, additional testing for specific inherited genetic disorders can be performed on the cells, if necessary. The preliminary test results are typically available in 3 working days. The preliminary test results are based on direct analysis of cells obtained from the CVS. The final test results are typically available within 10-14 days of the CVS.

The accuracy of CVS for the purposes of chromosomal analysis is comparable to amniocentesis, and is very high. An occasional error has been reported with both procedures. The accuracy of the testing is approximately 99%. In our experience, approximately 1% of the time, we obtain results that will not allow us to establish with certainty whether the baby’s chromosomes are normal. These situations are resolved on an individual basis and often require further testing such as examination of the parent’s chromosomes or amniocentesis. Similar unclear test results also occur after amniocentesis, but not as frequently. Very rarely, an insufficient amount of sample is obtained when CVS is attempted. In that event, another CVS may be offered or amniocentesis may be pursued.

Unlike amniocentesis, CVS will not give information regarding open neural tube defects, such as spina bifida or anencephaly. Neural tube defects occur in approximately 1 in 1000 livebirths. An open neural tube defect is an abnormal opening of the structure (neural tube) that forms the head, neck and spine. Women having CVS may wish to pursue maternal serum alpha-fetoprotein (MSAFP)screening test at 16 weeks gestation, if they wish to know whether they are at increased risk for having a baby with an open neural tube defect. This is a simple blood test that can be performed by your obstetrician.

With prenatal diagnosis, we attempt to rule out certain diseases. We are not able to state that the baby is healthy in every way. Thus, a report that the baby has normal chromosomes gives a high assurance that the baby does not have a chromosomal disorder. However, this does not guarantee that the baby is entirely normal. Approximately 3-5% of children are born with a birth defect, genetic disease or mental retardation. Many of these conditions, such as hearing loss and blindness, are not detectable through routine prenatal diagnosis or ultrasound, and they are not detected through a CVS.

Risks:

Although CVS is fairly safe, there is some risk associated with this procedure. Some women experience the following side effects after CVS:

• Cramping
• Spotting
• Infection
• Leakage
• Miscarriage

Miscarriages may occur following CVS. Many are unrelated to the CVS procedure; however, others represent complications due to the procedure. Studies in the United States and in Europe now suggest that the risk for CVS-related miscarriage is between 1/100 – 1/200. This is also a very small risk for uterine infection. We take precautions to prevent infection; however, if this should occur, it may require treatment.

To date, it is thought that there is not an increased risk for birth defects or suboptimal growth in babies following CVS. We are monitoring the outcomes of all pregnancies in which CVS has been performed to look for unexpected problems after testing.

Advantages:

The main advantage of CVS is that you can have this procedure during the first trimester of pregnancy. Many women choose CVS because they are able to have their test results before they are “showing” and can avoid the stress and anxiety associated with waiting until the second trimester. Therefore, CVS allows women to make decisions in the early stages of pregnancy. If a woman receives abnormal results after CVS and chooses to end the pregnancy, a termination is generally safer in the first trimester than if she waits for amniocentesis results.

Next Steps:

If you would like additional information regarding our experience with CVS or would like to schedule an appointment for a CVS, please contact our genetic counselors at 773-472-4900 or e-mail us.