Chorionic Villus Sampling
The placenta is located on the uterine wall and connects the fetus to the mother. Its role is to provide the fetus with all the energy and oxygen it needs. In the placenta there is an area of villi, each of which contains cells whose genetics is the same as the genetics of the fetus.
To examine the fetus’ chromosomes and to diagnose genetic diseases, such as Down Syndrome, in the early stage of pregnancy (in the first trimester), chorionic villus sampling (CVS) may be performed. During the test, which is performed between week 11 and until the end of week 13 of pregnancy, a small sample of chorionic villus tissue is collected. The sample is sent to the laboratory to extract DNA from the cells obtained in the sample and perform a genetic chip test, also known as Chromosomal Microarray Analysis (CMA) or other genetic tests (the cell sample is kept in the laboratory until the end of the test in case more genetic material is needed).
The test can be performed in two ways (depending on the location of the placenta), both under ultrasound guidance:
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Through the vagina
By inserting a catheter into the site where the placenta implanted, through which a small sample of tissue is extracted.
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Through the abdominal wall
By puncturing with a fine needle (similar to an amniocentesis), through which the sample is collected.
The decision on how to perform the test will be made on the day of the test by the medical staff, after the ultrasound scan is performed.
Purpose of the test
- To test fetal chromosomes: normally, a fetus has 46 chromosomes that are divided into pairs: 22 pairs of autosomal chromosomes (chromosomes that are not sex chromosomes) and 2 sex chromosomes - XX for female and XY for male. Mutations in several chromosomes can cause various chromosomal disorders. The test is performed with a molecular method that examines fetal DNA using the Genetic Chip Analysis (also known as CMA). It can facilitate a diagnosis of Down Syndrome (characterized by an extra copy of chromosome 21) and can detect other mutations attributed to extra or missing chromosomal segments.
- Diagnosis of a known hereditary disease in the family: if there is an increased risk of a certain disease, fetal cells will be tested to determine whether the fetus has the same disease. From a technical point of view, many tests may be performed on fetal cells and on the chorionic villi sample, but in practice only relevant tests are performed, depending on the assessed risk of delivering a child with a specific disease, and in accordance with the medical recommendation. For example, amniotic fluid can be tested to determine whether a fetus is vulnerable to Fragile-X Syndrome, but this test is only performed when the mother is a known carrier.
Date of the test
The test is performed between week 11 and until the end of week 13 of pregnancy.
The difference between chorionic villus sampling and amniocentesis
Both are invasive tests during which a biopsy is collected from the placental tissue or amniotic fluid, with the sample being sent to the laboratory for genetic testing of the fetus.
There are two main differences between the tests:
- A CVS is performed in the first trimester of pregnancy while the amniocentesis is only performed in the second trimester.
- With a CVS, you can perform all the tests that are performed with an amniotic fluid sample, except for a fetal protein test, which is used to diagnose defects in the fetal nervous system or tests for infections that are required by special referral.
Advantages of the test
Because the CVS is performed in the first trimester of pregnancy, the result is received at an early stage. Receiving a normal result indicates that no chromosomal mutations were detected in the fetus – in other words, stress will be alleviated, and the pregnancy can continue as scheduled. In case of an abnormal result (in the chromosomal test or the genetic test), it is possible to decide to terminate the pregnancy at this stage, a procedure performed by dilation and curettage under anesthesia.
Termination of pregnancy at later stages (week 19–21) is more complicated and largely performed by inducing labor. For this reason, when there is an increased risk of the fetus having a genetic or chromosomal disorder, a CVS may be considered. However, even when the chance of an abnormal fetus is low, some women prefer to undergo this test instead of the amniocentesis to obtain an early result and avoid the possibility of a late pregnancy termination.
Stages of the test
CVS is performed in several stages to protect your health as well as the health of the fetus.
Important to know
Important to know
The test usually takes several minutes and is not painful. However, many women experience discomfort during the procedure.
Recommendations after the test
- You should rest and avoid any exertion for 48 hours (you can receive approval for your place of work).
- You must also avoid sexual relations for 72 hours after the test.
Risks of the test
- Miscarriage: the risk of miscarriage following CVS is estimated to be 0.25% to 0.5% (one in every 200-400 tests). If you feel cramping (similar to contractions) in the lower abdomen, or vaginal bleeding, you should seek immediate medical attention from the treating physician or gynecology ER.
- Infection: an extremely rare complication since the procedure is performed under sterile conditions. If your body temperature rises after the test, if you experience chills or a general feeling of illness, you must seek immediate medical attention from the treating physician or go to the gynecology ER.
- Insufficient or bloody sample: on rare occasions, the test fails to collect sufficient quantity of cells to extract the fetal DNA required to perform the laboratory test, or alternatively - a bloody sample was collected. In these circumstances, you will need to decide whether to undergo another test or rely on the results of the screening tests.
- Inconclusive result: on rare occasions, results are inconclusive, and it is not possible to definitively determine whether or not the fetus is normal. In such a situation, it may be necessary to consider additional tests, such as amniocentesis or other examinations.
Receiving results
Test results are generally received within 3 weeks. On rare occasions, it is necessary to grow additional cells from the villi or additional tests are needed, which results in delays.
The test is 99% accurate, with 1% of tests returning with inconclusive results. In this case, you will be advised to undergo an amniocentesis or other tests. If an abnormal result is received (in other words, the fetus was diagnosed with Down Syndrome or other chromosomal or genetic mutations) you will be referred to genetic counseling to obtain all information and options available to you. In any case, the decision to continue the pregnancy is that of the parents.
Financing the test
Under the following circumstances, you will be entitled to funding for the CVS by the Ministry of Health or your HMO (in other cases, the test is privately funded):
- Every woman aged 32 and older: age will be calculated based on the last menstrual cycle. When the pregnancy is the result of an egg donation - age will be calculated according to the age of the donor, when pregnant from frozen embryos- the age will be calculated according to the date of fertilization.
- Every woman under the age of 32: the HMO will fund the test under the following circumstances:
- Results of the screenings (from the first trimester) indicating increased risk.
- Results of the nuchal translucency higher or equal to 3mm, and recommendations from genetic counseling to perform the test.
- Woman under the age of 32 who has another medical reason, or who received a medical recommendation following genetic counseling: will be entitled to Ministry of Health funding only after it is confirmed by the Genetics Department.
Important to know
Important to know
Eligibility for Ministry of Health funding of the test will only be provided if you have undergone the entire process without private funding (full or partial) and without funding of supplementary insurance from the HMO or private health insurance.
Exercise of the eligibility
If you are eligible for Ministry of Health funding of the test: you can directly contact one of the medical centers approved to perform the test (Hebrew).
Additional tests with CVS
There are other tests that can be privately performed on amniotic fluid that was collected during the test, such as:
- Tests that enable a rapid result to detect Down Syndrome (generally within 2–3 days), or a FISH test or a QF–PCR test.
- Growth of amniotic fluid cells collected during the test until the end of the pregnancy, so that if a finding is discovered later in the pregnancy that requires fetal testing, you will not have to undergo another diagnostic test.
- Exome test, a comprehensive genetic test of all functional units in genes, or genome test of the entire fetal genome. Today, these tests are not recommended during a pregnancy in which no significant abnormal findings were detected.
Information about these and other tests, which are performed on samples collected during the CVS, can be obtained during genetic counseling or at the site where you will undergo the test. In any case, it is important to emphasize that these tests are private and not funded by the Ministry of Health or the HMO.