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Chorionic villi are small structures in the placenta that act like blood vessels. These structures contain cells from the developing fetus. A test that removes a sample of these cells through a needle is called chorionic villus sampling (CVS).Chorionic villus sampling (CVS) is the removal of a small piece of placenta tissue (chorionic villi) from the uterus during early pregnancy to screen the baby for genetic defects
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CVS answers many of the same questions as amniocentesis about diseases that the baby might have. Diseases that can be diagnosed with CVS include Tay-Sachs, sickle cell anemia, cystic fibrosis, thalassemia, and Down syndrome. (Rh incompatibility and neural tube defects, however, can be diagnosed only through amniocentesis.) CVS can be done earlier in pregnancy than amniocentesis and can be done when there is not enough amniotic fluid to allow amniocentesis. However, it has some extra risks when compared with amniocentesis.
Why the Test is Performed
The test is a way of detecting genetic disorders. The sample is used to study the DNA, chromosomes, and enzymes of the fetus. It can be done sooner than amniocentesis, about 10 to 12 weeks after your last menstrual period. Test results take about 1 to 2 weeks, whereas amniocentesis results may take longer.
Chorionic villus sampling does not detect neural tube defects. If neural tube defects or Rh incompatibility are a concern, an amniocentesis will be performed.
This test can usually not diagnose problems in the way the body forms.
How the Test is Performed
CVS can be done through the cervix (transcervical) or through the abdomen (transabdominal). The techniques are equally safe when done by a provider with experience, although miscarriage rates are slightly higher when done through the cervix. The health care provider will use ultrasound to pick the safest approach and as a guide during sampling.
An abdominal ultrasound is performed to determine the position of the uterus, the size of the gestational sac, and the position of the placenta within the uterus. Your vulva, vagina, cervix, and abdomen are cleaned with an antiseptic such as Betadine.
The transcervical procedure is performed by inserting a thin plastic tube through the vagina and cervix to reach the placenta. The provider uses ultrasound images to help guide the tube into the appropriate area and then removes a small sample of chorionic villus tissue.
The transabdominal procedure is performed by inserting a needle through the abdomen and uterus and into the placenta. Ultrasound is used to help guide the needle, and a small amount of tissue is drawn into the syringe.
The sample is placed in a dish and evaluated in a laboratory.
What happens when the test is performed.
There are two ways that your doctor can perform CVS. Some patients have the sampling done through the vagina and cervix. Most patients have the sampling done through the abdominal wall. For both types of sampling, you lie on your back on an examination table and the doctor uses ultrasound to locate the fetus and the placenta.
If the sampling is to be done through the vagina and cervix, you place your feet in footrests and bend your knees up, as you would for a pelvic examination. A speculum (a device that looks like a duck-bill that can be opened and closed) is used to open the vagina so that your doctor can see inside. A long tube, much narrower than a straw, is inserted through the cervix and moved forward while your doctor watches on the ultrasound until it is next to the fetal side of the placenta. A small sample of the lining around the fetus is then pulled into the tube for testing.
If the sampling is to be done through the abdominal wall, your lower abdomen is cleaned with an antibacterial soap. In some cases, the doctor uses a small needle to inject a numbing medicine just under the skin, so that you do not feel the sampling needle. (Because the sampling needle does not cause much more stinging than the numbing medicine itself, not every doctor includes this step.) A hollow needle several inches long is inserted through the skin and muscle of the abdomen and through the wall of the uterus, to the edge of the placenta. This needle is held in place as a guide needle. A narrower needle is then inserted through the first needle and is rotated and moved inward and outward a number of times while a sample is collected into an attached syringe.
The fetal heart tones and the mother’s blood pressure and heart rate are checked at the beginning and end of the procedure. The whole procedure takes close to 30 minutes.
How to Prepare for the Test.
CVS can be done between the 10th and 13th weeks of pregnancy. Tell your doctor ahead of time if you have ever had an allergic reaction to lidocaine or the numbing medicine used at the dentist’s office.
Your health care provider will explain the procedure, its risks, and alternative procedures such as amniocentesis. Genetic counseling is recommended prior to the procedure. This will allow you to make an unhurried, informed decision regarding options for prenatal diagnosis.
You will be asked to sign a consent form before this procedure, and you may be asked to wear a hospital gown.
The morning of the procedure you may be asked to drink fluids and refrain from urinating to fill your bladder, which allows adequate visualization so the sample may be taken.
How the Test Will Feel
The ultrasound doesn’t hurt. A clear, water-based conducting gel is applied to the skin to help with the transmission of the sound waves. A handheld probe called a transducer is then moved over the area. In addition, your health care provider may apply pressure on your abdomen to find the position of your uterus.
The antiseptic cleansing solution will feel cold at first nd may irritate your skin if not washed off after the procedure. Some people are allergic to Betadine. Notify your health care provider if you are allergic to Betadine or if you have any other allergies.
Some women say the vaginal approach feels like a Pap smear with some discomfort and a feeling of pressure. There may be a small amount of vaginal bleeding following the procedure.
An obstetrician can perform this procedure in about 5 minutes, after the preparation
The risks of CVS are only slightly higher than those of an amniocentesis.
Possible complications include:
* Rh incompatibility in the mother
* Rupture of membranes
Signs of complications include:
* Excessive bleeding
* Excessive vaginal discharge
The risk of miscarriage and other complications from CVS is slightly higher than the risk from amniocentesis, although some parents feel that it is worth the extra risk to be able to makedecisions earlier in the pregnancy if the results show the baby has a health problem. There have also been some reports that suggest there is a very small risk of birth defects (abnormal limbs) in the fetus.
One particular difficulty with this test is that due to variability in the cells of the placenta (called mosaicism), occasionally you can have an abnormal test result even if the baby is normal and healthy. This might lead you to make decisions about pregnancy termination that you would not have made if you had better information.
Some women have vaginal bleeding after the procedure. Infection is uncommon.
Report any signs of complications to your health care provider.
CVS may also cause limb problems in the fetus. This risk appears to be very low (1 in 3,000) when CVS is performed after 10 weeks gestational ag
Time to know the result of the test
Chromosome analysis of the sample takes two weeks or more. The results of some tests may be available sooner.
A normal result means there are no signs of any genetic defects. However the test could miss some genetic defects.
Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
What Abnormal Results Mean
An abnormal result may be a sign of more than 200 disorders, including:
* Down syndrome
* Tay-Sachs disease
If your blood is Rh negative, you may receive RhoGAM to prevent Rh incompatibility.
You will receive a follow-up ultrasound 2 to 4 days after the procedure to make sure the pregnancy is proceeding normally.