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Chorionic Villus Sampling (CVS)


What is this test?

As in the case of amniocentesis, this test is also performed for the purpose of diagnosing genetic diseases in the fetus. In this case, however, the sample is taken from the fetal cells in the chorionic villi of the placenta.

How the test is performed?

At the beginning of the procedure, the physician examines the fetus and the placenta using the ultrasound transducer. Based on the location of the placenta and the week of pregnancy, the physician determines in which way the procedure is to be performed. There are 2 methods:
  • The first method, which is the more common, involves inserting a thin plastic tube through the vagina to the placenta. The entire procedure is performed under continuous ultrasound observation in order to guide the tube to the appropriate site.

  • The second method is usually performed in the advanced weeks of pregnancy and especially when the placenta is situated anteriorly. In this method the placenta is punctured through the abdominal wall.

In both cases, the chorionic villi containing fetal cells (and fetal genetic material) are taken, often with a number of maternal cells surrounding the placenta. If the number of villi is insufficient, the procedure is often repeated. In multiple gestations, a separate sample is taken from each fetus.

What the mother feels?

When the test is performed with a puncture, there is usually a slight pinprick pain similar to that felt when inserting a needle for removing blood from the arm. There is therefore no need for local anesthesia. Following the procedure, the mother should rest for half an hour to two hours in the place where the test was performed.

Sometimes, transient, localized pain in the region of the puncture is felt. The test may also result in a sensation of pressure as a result of contraction of the uterine muscles. This does not usually cause any problems and subsides after a short time with rest, preferably lying down. It is therefore advisable to rest for two days. It is extremely important not to lift objects or bend over for at least 3 days after the procedure.

What is undertaken in routine chorionic villus sampling?

Chromosome test, as in the case of amniocentesis. However, examination of the chorionic villi has one notable advantage: because the chorionic villi contain many cells undergoing division (the stage at which the chromosomes may be observed and examined), a direct chromosome test may be performed on the cells (before culturing). In this way, a result about the structure of the fetal chromosomes can be given after two days. The chromosomes are also examined after the cells from the chorionic villi have grown in culture (as in amniocentesis) in order to confirm the diagnosis.

The laboratory work is therefore more complicated for chorionic villus sampling. It requires the separation of the maternal cells from the fetal villi, direct processing, culturing the cells, and processing of the cultured cells.

Alpha-fetoprotein cannot be tested in chorionic villi, so every woman undergoing CVS must also undergo a blood test for alpha-fetoprotein between weeks 16 and 18.

What are the limitations of chorionic villus sampling?

All the limitations of amniocentesis apply here too.
Further limitations that may apply to chorionic villus sampling are:
  • Maternal contamination.
    Because the sample also involves maternal cells, there is a possibility that they may take over the culture, leading to the chromosome test being performed on the mother's rather than the fetus's cells. The risk of this happening decreases when the sample contains an adequate quantity of fetal cells and when the direct chromosome test (see above under "What is undertaken in routine chorionic villus sampling") is successful.

  • Mosaicism.
    It has been observed that the placenta contains many chromosomally abnormal cells (this is a fetal defense mechanism that -throws- abnormal cells into the placenta). Therefore, a result may be obtained stating there are both normal and abnormal cells in the chorionic villi. In these cases, amniocentesis must also be performed later, maybe together with other tests according to the recommendations of the genetic counselor.

When should chorionic villus sampling be carried out?

Chorionic villus sampling is usually carried out between the 10th and the 12th week of gestation. This allows the results of the chromosome analysis to be available between weeks 10 and 13. This is the main advantage of chorionic villus sampling. In the case of an abnormal fetus, the pregnancy can be terminated at an early stage by performing curettage, which is simpler than terminating a pregnancy by inducing early labor, as would be the case by the time that the result of an amniocentesis is received. Chorionic villus sampling can also be performed at later stages of pregnancy, especially if an urgent answer is needed. This can only be done, of course, if the placenta is accessible (frontal).

When will the result of the chromosome analysis be available?

As mentioned above, an initial result may be available in two days. A final result after culturing is obtained after 14-21 days.

Other tests that can be carried out on chorionic villi

As for amniocentesis, with a number of differences:
  • Diagnostic tests for intrauterine infection cannot be performed on chorionic villi cells.

  • Enzymatic and molecular genetic tests may be performed more efficiently. The amount of material obtained from a chorionic villus sample is greater than that obtained from amniocentesis, so that a result is usually available earlier (compared to amniocentesis), including for tests performed for non-chromosomal problems (enzymatic or molecular tests). The time required for growing the amniotic fluid cells in the culture before sending them for the specific test is saved.

  • In contrast to amniocentesis, precautions must be taken to avoid maternal contamination (testing maternal cells together with the chorionic villi), as this may cause misleading results. This risk can be avoided by sampling a sufficiently large quantity of chorionic villi, thoroughly cleansing the villi, and caution on the part of the laboratory testing the samples.

Risks associated with chorionic villus sampling

The risk of miscarriage occurring after chorionic villus sampling is approximately double that of amniocentesis (1 out of 100 to 1 out of every 150 procedures). The risk is increased only in the two days following the procedure. A slightly higher incidence of damage to the fetus' digits and limbs was reported in the past. This was later found not to be the case if the procedure is performed after the 10th week of gestation.

Ominous signs following chorionic villus sampling

  • Fever (over 37.7 degrees Celsius).

  • Painful contractions and a feeling of labor lasting more than 24 hours.

  • Breaking of the waters or bleeding.

In these cases, the family gynecologist must be contacted as soon as possible.

Who is advised to undergo chorionic villus sampling?

A partial list of indications is provided below:
  • Any case that requires an urgent answer.

  • Couples at high risk of having a child with a genetic disease as a result of the known gene carrier status of one or both of the parents (depending on the inheritance pattern of the disease). In these cases, the risk that a problem will be found in the fetus is higher, so as early a diagnosis as possible is important.

  • When one of the parents has a balanced chromosomal change (translocation) and there is a relatively high risk of an unbalanced chromosomal problem in the fetus.

  • All the reasons specified for amniocentesis for women who do not want to wait for amniocentesis.

  • Recently, tests identifying women at an early stage of pregnancy who are at increased risk for having a child with Down syndrome have been added based on the results of nuchal translucency and biochemical tests performed in the first trimester. In cases where the risk for Down syndrome has been calculated as being very high in the first trimester, it is worth considering this test rather than amniocentesis

Umbilical cord puncture (cordocentesis)

In this test, a fetal blood sample is taken by puncturing the umbilical cord under ultrasound guidance. As a relatively quick chromosome test can be performed on the cells of the blood obtained, since there is no need to grow cells as in the case of amniocentesis, this test can give a quick result in cases where urgent problems arise in the later weeks of pregnancy. An example would be a suspected chromosomal problem arising following an abnormal ultrasound test finding. It also allows additional laboratory tests to be carried out on the fetal blood sample, such as antibody levels indicating exposure to viral infection, or tests for blood clotting disorders such as hemophilia.

The use of this test has decreased over the years following advances in laboratory methods that enable the same information to be obtained from amniotic fluid, since sampling this is both less complicated and less dangerous. However, in certain cases where amniocentesis does not provide a satisfactory answer, this test can be performed, despite its risk being somewhat higher than that of other invasive tests (the risk of miscarriage is one out of every 100 procedures).
 
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