If a pregnant woman is less than 12 weeks pregnant, Chorionic Villus Sampling (CVS) may be performed, thereby offering earlier results than amniocentesis. The following is some basic information about CVS.

Who should consider CVS?

CVS should be considered by women age 33 years or over, individuals who have had a child with a chromosomal abnormality, individuals who have a chromosomal translocation, and couples at risk for a prenatally identifiable genetic disorder (e.g., hemophilia or sickle cell disease).

CVS may not be appropriate for individuals with a family history of neural tube defects (spina bifida or anencephaly).

When is CVS performed?

CVS is traditionally performed between 10 -12 weeks after a woman's last menstrual period (during the first trimester). Women who are between 15-18 weeks pregnant may wish to consider amniocentesis.

How is CVS performed?

There are two methods for obtaining chorionic villi. For many women, either method may be appropriate. First, an ultrasound evaluation is performed to locate the developing placenta and to date the pregnancy. Often, the placental location determines which method of CVS is more appropriate. There are certain other obstetrical considerations which may make one method preferable, including uterine anatomy and vaginal infections.

• Transcervical CVS: A thin catheter (hollow tube) is inserted through the vagina and cervix. Ultrasound is used to guide the catheter to the edge of the developing placenta. A small amount of placental tissue is removed and sent to be analyzed.
• Transabdominal CVS: This procedure is similar to amniocentesis. Ultrasound is used to select a site for inserting a needle. After a local anesthetic is given, a needle is inserted, with ultrasound guidance, through the abdomen and placed at the placenta. A small amount of tissue is removed and sent to be analyzed.

Do the procedures hurt?

Most women compare the transcervical CVS to a pelvic exam, which may be somewhat uncomfortable. The transabdominal CVS is very similar to an amniocentesis.

What are the risks of CVS?

Most miscarriages occur early in pregnancy regardless of whether any invasive testing has been performed. A test like an amniocentesis carries a small increased risk of having a miscarriage (about half of one percent) and the risk after a CVS procedure is slightly higher (about half to one percent). To help keep this risk as low as possible, women are given precautionary instructions after the procedure.

Several years ago reports appeared in the medical literature regarding women who had undergone CVS and had subsequently delivered infants with limb abnormalities including missing fingers and toes. This association appeared to correlate with, but was not limited to, CVS performed earlier than nine weeks from the onset of their last menstrual period. Recent studies have been performed to provide additional data concerning this issue.

In a recent study conducted by the Center for Disease Control, there was no overall increased risk of limb abnormalities among infants whose mothers had undergone CVS compared to infants whose mothers did not undergo this procedure. However, there was an increased risk for a specific abnormality involving missing fingers or toes (aka "transverse digital deficiency"), with the absolute risk being approximately 1 in 3000 births. This type of birth defect also occurred in infants whose mothers had undergone amniocentesis or who had no form of prenatal testing.

By contrast, an international World Health Organization CVS registry of 138,000 infants born after CVS from 19 medical centers showed no significant difference in the overall frequency or pattern of limb abnormalities compared to the background population frequency.

Based upon currently available information, we conclude the following:

1. CVS is a relatively safe and accurate procedure and is considered an acceptable alternative to genetic amniocentesis.
2. CVS is best performed at 10-12 weeks pregnancy based on the beginning of the last menstrual period.
3. Although it is uncertain whether there is an increased risk of limb abnormalities following CVS, we believe it is important for our patients to understand that this controversy has been raised and that the absolute risk may be in the order of 1 in 3,000 births. Unfortunately, between 2%-3% of all children are born with a birth defect regardless of whether or not a woman had prenatal testing during her pregnancy.

Our physicians at the Baylor College of Medicine have been performing CVS since the center's inception in 1984. It is felt that at our center, as well as others of similar experience, the risk of limb abnormalities (approximately 1 in 3,000) needs to be placed in proper perspective and weighted against the information that first trimester prenatal testing can provide.

What is analyzed from CVS?

The chorion is the outer membrane of the sac which surrounds the fetus. Early in pregnancy, the chorion is covered by fingerlike projections called villi. In one area, the villi will implant in the uterus to develop into the placenta and the remainder of the villi disappear. The chorionic villi develop from the same fertilized egg as the fetus so the genetic material should be the same. The cells from the villi are cultured (grown) in the laboratory. Chromosomal analysis and other special genetic testing can be performed on these cells.

What can be detected through CVS?

CVS detects chromosomal abnormalities, such as the kind that causes Down syndrome. If indicated, some genetic alterations responsible for genetic disorders may be identified through DNA or enzyme analysis. The CVS cannot detect neural tube defects such as spina bifida. Therefore, it is recommended that all women who undergo CVS consider having their blood drawn at 15-18 weeks of pregnancy for a spina bifida screening test (maternal serum AFP assay). This may be arranged through the patient's obstetrician. Many women also wish to pursue a level II ultrasound at 16-20 weeks for further information regarding the physical development of the fetus. Prenatal testing can only identify some genetic errors for which a couple is known to be at risk. Unfortunately, there are many conditions for which prenatal testing is not yet available.

While the prenatal CMA test is very accurate, not every genomic abnormality (genetic defect) can be detected by a prenatal CMA test. For some conditions, genomic gains or losses at a particular locus may represent only a certain percentage of the genetic changes associated with that given disorder. For instance, in some disorders, 99% of the cases may be detected by the test, while for others the detection rate may be 70% or less. No method of prenatal testing can guarantee a baby will be born without birth defects or genetic disorders, nor does every genetic alteration have profound clinical consequences. For this reason, results from your tests should be thoroughly discussed with clinicians, genetic counselors and other qualified medical personnel.

How accurate are the results from CVS?

Chromosome results from CVS are greater than 99% accurate. However, these results must occasionally be clarified through blood tests on the parents, ultrasounds, amniocentesis, or fetal blood sampling. Maternal serum AFP screening is greater than 80% accurate in detecting neural tube defects.

When will test results be available?

CVS results will be available approximately two weeks after the procedure. Special studies for other genetic disorders may take longer. Maternal serum AFP results are usually available one week after blood is drawn.

How many appointments are needed?

All women will be scheduled for a genetic counseling session to discuss the procedure in detail, determine whether additional genetic testing is appropriate, and ensure that all concerns have been addressed. Those selecting CVS are scheduled for the procedure between 10-12 weeks. These two appointments may be scheduled on the same day if desired.

What are the advantages of CVS?

The greatest advantage of CVS is timing. Since CVS is performed during the first trimester of pregnancy, the results from the CVS are available earlier (12-15 weeks) than are amniocentesis results (17-20 weeks). If a genetic abnormality is found, families must then decide whether they wish to continue the pregnancy. First trimester pregnancy terminations can be safer, more easily performed, less expensive, and generally less emotionally difficult than second trimester terminations.