What is chorionic villus sampling?
Chorionic villus sampling (CVS) is a method of prenatal diagnosis, which is a way to directly test for genetic conditions in the fetus.
During the CVS test, a small sample of cells (called chorionic villi) is taken from the placenta where it attaches to the wall of the uterus. Chorionic villi are tiny parts of the placenta that are formed from the fertilized egg, so they typically have the same genes as the fetus.
If you have certain risk factors, you may be offered CVS as a way to detect genetic conditions during early pregnancy. Whether you choose to have the test is up to you.
What diseases or disorders can chorionic villus sampling identify?
CVS can help identify chromosomal problems such as Down syndrome or other genetic diseases such as cystic fibrosis, Tay-Sachs disease, and sickle cell disease. CVS does not detect birth defects. An ultrasound performed later in the pregnancy (18-20 weeks) will usually check for most birth defects. If you have CVS, you may want to consider having a blood alpha-fetaprotein (AFP) test later in pregnancy to screen for neural tube (brain, spine, or spinal cord) defects.
What are the benefits of chorionic villus sampling?
CVS can be done earlier in pregnancy than amniocentesis. Getting this kind of information early allows a woman to make choices in the beginning stage of her pregnancy. If a woman chooses to end her pregnancy after receiving abnormal test results, the pregnancy termination will be safer than if she waits for amniocentesis results.
What are the potential risks of chorionic villus sampling?
CVS may carry a slightly higher risk of miscarriage than amniocentesis, since the procedure is done in early pregnancy. The risk for pregnancy loss (miscarriage) related to a CVS is about 1 in 200. Infection may also be a complication. Rare cases of limb deformities in infants have been reported, but most of these have been related to doing the CVS prior to 10 weeks gestational age.
Who should have a chorionic villus sampling test?
Testing should be offered to:
- Pregnant women who will be 35 or older on their due date (the risk of having a baby with a chromosomal problem such as Down syndrome increases with the age of the woman)
- Couples who already have had a child with a known genetic condition or have a family history of a genetic condition
- Pregnant women at risk to have a child with a particular genetic condition based on abnormal or positive screening results
How is chorionic villus sampling performed?
CVS requires genetic counseling, including a detailed talk with a counselor regarding the risks and benefits of the procedure.
At the time of initial consultation and counseling, an ultrasound exam will be performed to confirm gestational age (the development stage of the embryo) and the location of the placenta. This is done so that CVS can be performed at the appropriate gestational age, usually 11-13 weeks from the first day of a woman's last menstrual period.
There are two ways to collect chorionic villi from the placenta: through the vagina or through the abdomen:
- To collect cells through the vagina, a speculum is inserted (in the same way as a Pap test). Then a very thin, plastic tube is inserted up the vagina and into the cervix. With ultrasound, the tube is guided up to the placenta, where a small sample is removed.
- To collect cells through the abdomen, a slender needle is inserted through the woman's abdomen to the placenta, much like amniocentesis.
Results and Follow-Up
The sample of chorionic villi is sent to a lab where the cells are grown in a special fluid and tested a few days later. Culture results will be available within 7-10 days.
Preliminary results may be available in 2-3 days. A genetic counselor will notify you of the results and answer any questions that you may have about them.
Here are words that are important to know when thinking or talking about CVS:
Amniocentesis: a procedure in which a small amount of amniotic fluid is taken from the sac surrounding the fetus and then tested.
Amniotic fluid: the water in the sac surrounding the fetus in the mother's uterus.
Chorionic villi: microscopic, finger-like projections that make up the placenta.
Chromosomes: structures located inside each cell in the body containing the genes which determine a person's physical make-up.
Embryo: a term that describes the developing baby from the moment of conception to the end of the 8th week of pregnancy.
Fetus: a term that describes the developing baby from the end of the 8th week after conception to the moment of birth.
Placenta: Tissue that connects the woman and fetus. It provides nourishment to the fetus and takes away wastes from the fetus.
Ultrasound: A test in which sound waves are used to examine internal structures. During pregnancy, it can be used to examine the fetus.
Uterus: A muscular organ that is located in the female abdomen and contains and nourishes the developing embryo and fetus during pregnancy.