Chorionic Villus Sampling
CVS is a good option in some pregnancies but not in every pregnancy, and the decision to test is yours alone. It is performed between 10 and 14 weeks, so it is important that parents who would like to have this testing done contact their obstetrician or midwife early in the pregnancy to schedule the procedure.
Any woman who is pregnant, regardless of her age, is at risk to have a child with a chromosome condition and therefore can choose to have CVS. However, there are certain factors that put individuals at higher risk, such as age, family history, or an abnormal ultrasound finding.
In the past, it was routine in this country to offer CVS to women who were age 35 and older at delivery. This practice is now changing. CVS and prenatal screening tests are now available to all pregnant women.
CVS looks at a baby's chromosomes by testing the placental cells. 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.
CVS involves taking a small piece of the placenta, and takes only a few minutes. It can be done in one of two ways.
- The first way is for the doctor to put a needle through the mother's abdomen to reach the placenta.
- The second way is for the doctor to put a catheter (a plastic tube) through the mother's cervix to reach the placenta.
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.
- Women with an Rh-negative blood type would need a Rhogam shot within 72 hours after undergoing CVS to avoid certain problems.
- 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.
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.
This test is both relatively safe and accurate, but there are some risks and side effects associated with the procedure.
- The risk of miscarriage as a complication of CVS is about 1 in 100 (1 percent). Often we can not determine the exact cause of a miscarriage, and it is possible that an underlying fetal problem was present. However, sometimes we can identify a cause, such as an infection.
- 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.
All pregnancies have about a three- to five-percent chance of resulting in a child with some type of condition that our studies would not detect. A normal CVS result does not mean that your baby is healthy in every way. Babies can still be born with a birth defect, inherited disease or mental retardation, even with normal chromosomes.
If you have any further questions about CVS, or if you are having trouble deciding if you want this test, speak with your pregnancy care provider or a genetic counselor.
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