Chorionic Villus Sampling (CVS)

Chromosomes are the structures in our cells that carry our genetic information (genes). Typically, we have 46 chromosomes in each of our cells. When a baby is conceived, he or she inherits half of their chromosomes from the egg and the other half of their chromosomes from the sperm. The most common chromosome problems, such as Down syndrome, occur when an extra or missing chromosome is present. Many of these chromosome problems can be detected by CVS.

Each chromosome carries many genes. Genes control our growth and development, and determine many of our traits. If a family is known to be at risk for a disorder caused by an error (mutation) in a single gene, CVS can also often be used to detect this disorder.

As a woman gets older, her chance of having a pregnancy with a chromosome abnormality increases. However, there is always a risk of having a pregnancy with a chromosome abnormality, even when a woman is very young. Some of these abnormalities can have severe effects on development, some can be milder, and some may be unpredictable in their effects. CVS can test for the majority of chromosome problems, including Down syndrome.

The main advantage of CVS is that it can be performed during the first trimester of pregnancy. This allows for earlier counseling and decision-making, often before a woman is visibly pregnant. Should pregnancy termination be an option for the couple, this procedure is typically safer in the first trimester.

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.

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 on the day of the procedure, during the ultrasound examination. A thorough ultrasound examination is always performed prior to and during the procedure.

• Transcervical CVS
  A speculum is inserted into the vagina, and a catheter (thin, hollow plastic tube) is guided through the cervix and into the placenta. A small piece of the placenta is then removed.
• Transabdominal CVS
  A slender needle is inserted through the abdomen and into the uterus and placenta. A small piece of the placenta is removed by applying gentle suction to the syringe attached to the needle.

Both methods typically require only a few seconds or minutes. The entire process typically requires about 20 minutes, including the ultrasound prior to the procedure. Most women report minimal discomfort associated with both procedure types.

Once the placental cells are obtained, they are studied under a microscope. The chromosomes are counted, and the gross structure of the chromosomes is checked. If necessary, additional testing for specific inherited genetic disorders can also be performed on the same cells.

The preliminary test results are typically available within 3 working days. The final test results are typically available within 2-3 weeks of the CVS. For families who are at risk for a specific genetic disorder, the timing of the test results will vary depending upon the laboratory used.

The accuracy of CVS for the purposes of chromosome analysis is very high (approximately 99%), and is comparable to amniocentesis. Approximately 1-2% of the time, results may be inconclusive, and a follow up amniocentesis or a blood test may be recommended to clarify the results. Unclear test results can also occur after amniocentesis, but they are less common. Very rarely, an insufficient amount of sample is obtained when CVS is attempted. In that event, another CVS or an amniocentesis may be recommended.

Unlike amniocentesis, CVS does not provide information about open neural tube defects (where there is an abnormal opening of the structure that forms the head, neck, and spine), such as spina bifida. Neural tube defects occur in approximately 1 in 1000 live births, and are not related to a woman’s age. For women having CVS, maternal serum alpha-fetoprotein (MSAFP) screening is recommended at 16 weeks gestation, in order to determine if there is an increased risk for a neural tube defect. MSAFP screening is a simple blood test that can be performed by your obstetrician.

Prenatal diagnosis can rule out certain chromosomal or genetic diseases. It is not able to determine whether or not a baby is healthy in every way. Therefore, while a normal chromosome result provides a high assurance that the baby does not have a chromosomal disorder, it does not rule out every possible condition or illness. Approximately 3-5% of children are born with a birth defect, genetic disease, or cognitive disability. Many of these conditions, such as hearing loss, blindness, or autism, are not detectable through routine prenatal diagnosis (CVS or amniocentesis) or ultrasound.

Although CVS is typically safe when performed by an experienced physician, there are some risks associated with the procedure. Some women may experience the following side effects after CVS:

• Cramping
• Spotting or bleeding
• Infection
• Leakage of amniotic fluid

CVS poses a small risk of miscarriage. Most miscarriages that occur following a CVS are unrelated to the procedure; however, some may represent complications caused by the procedure. Our experienced physician has found the risk for CVS-related miscarriage to be between 1/500 – 1/1000.

As long as CVS is performed after 10 weeks gestation, it does not appear to cause an increased risk for birth defects or development problems. We are always monitoring the outcomes of pregnancies in which CVS has been performed, in order to look for unexpected problems after testing.