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Chorionic villous sampling, or CVS, is an antenatal test that can rule out or detect certain genetic abnormalities in the foetus. This involves taking a small amount of placental tissue. The chorion is the portion of fetal membrane that eventually forms the fetal side of the placenta. It contains chorionic villi, which are small finger-like projections that are snipped or suctioned off during the procedure. Since chorionic villi are of fetal origin, examining samples of them can give the genetic make-up of the fetus. |
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| When is chorionic villous sampling done? |
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| CVS is usually performed at 10 to 12 weeks of pregnancy. However, it can also be performed later in pregnancy. CVS is used to detect many of the same abnormalities as an amniocentesis. Instead of sloughed fetal cells, CVS examines placental cells, which are genetically similar to the fetus. Since CVS can be done earlier in the pregnancy, it may be preferable for women who need quick answers about their baby's health. However, CVS carries a higher risk of miscarriage compared to an amniocentesis. |
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| Why is CVS performed? |
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| The CV sample is used to study the DNA, chromosomes, and enzymes of the fetus. It may be offered to pregnant women who have an increased risk of having a baby with a genetic birth defect, such as in the following: |
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- Those who will be 35 or older on their delivery date.
- Those who have had a previous pregnancy or baby with a certain genetic abnormality.
- Those who have a family history of a certain genetic disorder.
- Those with a positive first trimester screening (FTS) for Down syndrome
- Those with an abnormality found on ultrasound scan such as increased NT (nuchal translucency).
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| In Malaysia, CVS is commonly done for women with the risk of having a baby with alpha or beta Thalassaemia major. These are blood disorders, which can severely affect the baby. |
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| What are the risks of CVS? |
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| Although CVS can detect many of the same congenital problems earlier in the pregnancy, it carries a 1% to 2% risk of miscarriage. A recent audit at FMGC showed a fetal loss rate of 1%. The degree of risk for the individual is often related to the skill and experience of the doctor performing the procedure. |
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| CVS may be chosen as the method of prenatal testing for those unable to have an amniocentesis, and those who may consider a therapeutic pregnancy termination in case of birth defects; if the fetus has serious complications an earlier abortion is often easier and safer than a second trimester abortion or possible second trimester miscarriage. |
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| The following is a list of possible complications: |
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- Rupture of membranes (rupture of the amniotic membrane)
- Miscarriage
- Infection
- The mother developing Rhesus isoimmunisation
- Contamination of the sample with maternal cells
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| How can I prepare for the test? |
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| The doctor will explain the procedure, its risks, and alternative procedures, including amniocentesis. Genetic counselling is also provided before the procedure, to discuss the pros and cons of testing and how the test results may affect you and your family. You may be asked to drink fluids and refrain from urinating for a few hours before the test. A full bladder can help in the proper positioning of the uterus, which allows adequate visualisation for the sample to be taken. |
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| Before having CVS, you and your husband/partner will be asked to sign a consent form. The test does not require an overnight stay in hospital. There are no food or fluid restrictions and it does not require a general anaesthetic. |
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| How is the test performed? |
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| There are two ways to perform CVS: through the abdomen, trans-abdominal CVS or through the cervix (neck of the womb), trans-cervical CVS. An ultrasound examination is performed prior to the CVS to document the fetus is viable and to establish its gestational age. The placenta is also located, so that the best approach for the test can be determined. |
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| For the trans-abdominal approach, the doctor will clean the skin with some antiseptic and then a local anaesthetic is given through the skin of your abdomen using a fine needle to numb the site. A needle is then inserted through the skin into the placenta in the womb, under ultrasound guidance. You will feel some pain momentarily as the needle enters the uterus. A smaller second needle is then inserted through the first needle. With a forward and backward movement, a small amount of placental tissue is then drawn up into a syringe. Using this 2-needle technique, repeated samples can be taken without causing any more discomfort. When adequate tissue is obtained, both needles are then withdrawn and a little plaster is applied to the site. The sample is then placed in a sample dish and prepared for evaluation in the laboratory. |
| Under ultrasound guidance, a thin plastic or metal cannula (tubing) is then passed through the cervix to reach the placenta. Using a forward and backward movement, some placental tissue is drawn up into a syringe. The cannula is then removed when adequate tissue is obtained. The sample is then placed in a sample dish and prepared for evaluation in the laboratory. Some women experience no pain with the trans-cervical CVS while others report feeling a pressure or cramping similar to a pelvic examination. |
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| Neither of these procedures requires general anaesthesia. Your husband, partner, relative or close friend may stay with you during the test if you wish. An ultrasound scan is done after the test to ensure that the fetus is okay. |
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| The do's and don'ts |
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| Strenuous activity should be avoided for 24 to 48 hours and it is wise to refrain from sexual intercourse for one week. You will be given two days off work. |
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| CVS is done on an outpatient basis and does not require hospitalization. It takes only a couple of minutes. You do not require any close monitoring or observation afterwards and you would be allowed home after the procedure. |
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| Mild spotting and cramps is not uncommon for up to one week following the CVS, particularly the trans-cervical CVS. A sanitary pad rather than a tampon should be used. Significant bleeding (with clots), flu-like symptoms, fever, abdominal pain, or leakage of fluid should be reported to your doctor immediately as these may be signs of an infection or early miscarriage. |
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| Those who are Blood Group Rhesus Negative will be given an injection of RhoGam (Anti D) to prevent the complication of Rhesus isoimmunisation within 36 hours. |
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| When will the results be ready? |
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| This depends on what abnormality is being tested. A chromosome analysis will take 10 to 14 days. Tests for genetic abnormalities may be obtained within the week such as a-Thalassaemia. ?thalassaemia, on the other hand may take up to 2 weeks. You will be informed as soon as the result is ready. |
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| CVS can accurately rule out or detect certain genetic abnormalities in more than 99% of the time. Very occasionally, the results of the test are inconclusive. Women with inconclusive CVS results will be advised on further tests that are necessary. |
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| Trans-abdominal cvs: |
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Local anaesthetic is given to numb the area |
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Double needle technique is used. |
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| The procedure is done under ultrasound guidance, so that the doctor knows exactly where to place the needle. |
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| Trans-cervical cvs: |
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After cleaning the vagina area, a small cannula or tubing is inserted through the cervix to reach the posterior placenta. |
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| The placental sample is then taken under ultrasound guidance. |
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