Chorionic villus sampling often referred to as CVS, is a diagnostic test for identifying chromosome abnormalities and other inherited disorders. This test may be recommended by your health care provider if you or your partner has family medical histories that reveal potential risks. The main advantage of CVS is you receive results early in your pregnancy.
How is the chorionic villus sampling (CVS) performed?
CVS is a diagnostic procedure which involves removing some chorionic villi cells from the placenta at the point where it attaches to the uterine wall.
There are two ways samples are collected:
Transcervical: An ultrasound guides a thin catheter through the cervix to your placenta. The chorionic villi cells are gently suctioned into the catheter. This is the most common method.
Transabdominal: An ultrasound guides a long thin needle through the abdomen to your placenta. The needle draws a sample of tissue and then is removed. This procedure is similar to that of amniocentesis.
Before the test, the doctor uses ultrasound to figure out which way to do the CVS. The simplest route to the placenta is usually the safest for the mother and baby. Usually the doctor will need only one try to obtain the sample, except in the case of twins where one procedure may be needed for each baby. In some cases, a second attempt will be needed for a single baby to obtain enough placental cells. Rarely, it is not possible to obtain enough placental cells to do the testing or it is not possible to reach the placenta to obtain a sample.
The CVS procedure collects larger samples and provides faster results than amniocentesis. Results may be received between one to seven days.
When is chorionic villus sampling (CVS) performed?
CVS is usually performed between 10 and 13 weeks from your last menstrual period. CVS may be chosen over amniocentesis because it may be performed earlier in the pregnancy. If you would like to have this testing done contact your doctor early in your pregnancy to schedule the procedure.
What does the test look for?
Chorionic villus sampling looks at a baby’s chromosomes by testing the placental cells for chromosome abnormalities. The placenta and the baby have the same genetic information, so the laboratory can grow and then study the placental cells to learn about the baby. Chromosomes are small packages of DNA that contain all of our genes, which provide the instructions for human development. In most individuals, each cell contains a total of 46 chromosomes. CVS can detect if the baby has an extra or missing chromosome or large changes to chromosome structure. Changes to chromosomes may lead to serious birth defects and/or mental retardation.
- CVS tissue can be used to test for certain inherited genetic diseases. If a couple is at known risk to have a child with a certain genetic condition such as cystic fibrosis, sickle cell disease, or Tay-Sachs disease, it is possible to test the pregnancy for this condition using CVS.
- Down syndrome, or trisomy 21, is the most common chromosome condition detected. It is caused when there is an extra 21st chromosome in each cell, causing a total of 47 chromosomes. People with Down syndrome have mild to moderate mental retardation and may have certain birth defects.
- Trisomy 18, or Edward syndrome, is less common than Down syndrome. It is caused when there is an extra 18th chromosome instead of the 21st for Down syndrome. Few babies with Trisomy 18 survive to birth. Trisomy 18 causes mental retardation, heart defects, very poor growth and other problems.
- CVS does not test for birth defects including heart defects or spina bifida. Couples who desire screening for spina bifida may wish to consider having a test to measure the alpha fetoprotein chemical in the mother’s blood. You may also want to consider an ultrasound, which can detect most cases of spina bifida between 18 and 20 weeks of pregnancy.
This test is different from amniocentesis in that it does not allow for testing for neural tube defects.
Chorionic villus sampling also provides access to DNA for paternity testing prior to delivery. DNA is collected from the potential father and is compared to DNA obtained from the baby during chorionic villus sampling. The results are accurate (99%) for determining paternity.
Processing the test
- On average, results take about 10-14 days.
- In situations where information is needed more quickly, preliminary results of the CVS may be available in 48 hours.
- Sometimes the results will take longer than expected if the placental cells do not grow in the laboratory as quickly as expected.
- Very rarely, placental cells will not grow at all and the results of the CVS or any special tests that were ordered with the CVS will not be reported. In this case, a second CVS may be needed.
- If the pregnancy is beyond 14 weeks, an amniocentesis could be an option.
- If any special genetic testing was done with the CVS, the timing for these results will depend on the condition being tested for and may take over a month.
Accuracy of CVS
The accuracy of this test for the most common problems is approximately 99 percent. If any special genetic testing was done on the CVS sample, the accuracy will depend on the condition being tested and on the laboratory doing the testing.
What do chorionic villus sampling (CVS) results mean?
CVS is a diagnostic test that detects chromosome abnormalities and genetic disorders with high levels of accuracy (98-99%). Although the probabilities of identification are high, this test does not measure the severity of these disorders. This test does not help identify neural tube defects.
Are there risks or side effects to the mother or baby?
Although CVS is considered to be a safe procedure, it is recognized as an invasive diagnostic test that does pose potential risks. Miscarriage is the primary risk related to CVS occurring 1 out of every 100 procedures.
CVS is not recommended for women who:
- Have an active infection (i.e. STD)
- Are carrying twins
- Have experienced vaginal bleeding during pregnancy
Transcervical CVS is not recommended for women who:
- Have uterine fibroids
- Have a tilted uterus which impedes the catheter
Since CVS involves entering the uterus with either a needle or a catheter, women having CVS may experience discomfort, usually described as pressure or cramping. Some cramping for several hours after the procedure is common. If the procedure is done using a needle, soreness, tenderness, or bruising at the site of the needle entry may occur. If the procedure is done using a catheter, light vaginal blood spotting may occur.
Contact your healthcare provider if these symptoms remain or get worse.
You should also contact your healthcare provider if you experience:
- Leaking of amniotic fluid
According to the Mayo Clinic, there is a 1% chance of getting false positive results. A false positive occurs when the test indicates that the fetus has an abnormality, but it actually does not.
What are the reasons to test or not test?
The reasons to test or not test vary from person to person and couple to couple.
Performing the tests and confirming the diagnosis provides you with certain opportunities:
- Pursue potential interventions that may exist
- Begin planning for a child with special needs
- Start addressing anticipated lifestyle changes
- Identify support groups and resources
- Make a decision about carrying the child to term
Some individuals or couples may elect not to pursue testing or additional testing for various reasons:
- They are comfortable with the results no matter what the outcome is
- Because of personal, moral, or religious reasons, making a decision about carrying the child to term is not an option
- Some parents choose not to allow any testing that poses any risk of harming the developing baby
It is important to discuss the risks and benefits of testing thoroughly with your healthcare provider. Your healthcare provider will help you evaluate if the benefits from the results could outweigh any risks from the procedure.
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Compiled using information from the following sources:
1. Mayo Clinic Complete Book of Pregnancy & Baby’s First Year. Johnson, Robert V., M.D., et al, Ch. 11.
2. Williams Obstetrics Twenty-Second Ed. Cunningham, F. Gary, et al, Ch. 13.