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Auxiliary Invasive Tests


Amniotic fluid test (amniocentesis)

What is amniocentesis?

Amniocentesis is a test performed for the purpose of diagnosing genetic diseases (mainly chromosomal disorders) in the fetus. It is aimed at determining whether there are any abnormalities in the number and structure of the chromosomes, which contain all the genetic material of the individual. The test can also be used to identify other conditions such as abnormalities in the alpha-fetoprotein level in the amniotic fluid, which is indicative of a defect in the closure of the fetus' neural tube (NTD).

There are other diseases that can also be identified by examination of amniotic fluid using molecular genetic or enzymatic/biochemical methods, etc. However, such tests, which are available today for an increasing number of conditions, are not routinely performed and are undertaken only in cases in which both members of the couple carry defective genes that have already been identified in their blood and that may be transferred to a fetus, causing the disease. In other words, a couple at high risk for bearing a child with a specific genetic disease that can be tested for by examination of the amniotic fluid will also be eligible to undergo this test.

How is the amniotic sac punctured to obtain a sample of amniotic fluid?

At the beginning of the procedure, the physician looks at the fetus, the placenta and the amniotic fluid using the ultrasound transducer. He then thoroughly disinfects the skin of the abdomen in order to prevent infection. For the puncture, a fine needle is inserted through the abdominal wall into the fluid surrounding the fetus. The insertion of the needle is guided using the ultrasound transducer, and is directed into the site with abundant fluid in the area of the fetus' feet. This means that no damage will be caused to the fetus. A quantity of fluid sufficient for 2 ampoules (usually approximately 20 cc), which does not endanger the fetus, is drawn through the needle. This is replenished in a few hours. In multiple pregnancies (twins or more), where each fetus is in its own sac, fluid is drawn from each sac separately and is handled and examined separately.

What does the mother feel?

There is usually only slight pain associated with the puncture, similar to that experienced when inserting a needle for taking blood from ones arm. There is, therefore, no need to anesthetize the site.

Immediately after the puncture the woman must rest for half to two hours. Sometimes, transient, localized pain in the puncture region is felt. There may also be a sensation of "pressure" as a result of contractions of the uterine muscles. This does not usually cause any problems and subsides after a short time with rest, preferably lying down, and it is therefore advisable that the woman rests for two days. It is extremely important that she does not lift objects or bend over for at least 3 days after the amniocentesis.

What is done with the amniotic fluid?

  • The fluid contains cells that have sloughed off the fetus skin. These are incubated on a number of dishes in the laboratory under special conditions. Each cell grows into a colony of dividing cells, and usually after approximately 10 days there are sufficient colonies for testing. Then the surfaces are processed in order to fix the dividing cells. At the time of cell division, and only at this stage, the genetic material in the chromosomes may be observed under a microscope. The laboratory workers look at the chromosomes of a particular cell under the microscope in order to check whether the number and structure are normal. The chromosomes can also be photographed and the resulting photographs examined. It is of course necessary to look at several cells from a number of dishes (a number of "sources") before giving an answer. The most common chromosomal disorder is Down syndrome, but of course any change in the number or structure of chromosomes may cause severe problems.

  • The fetal protein (alpha-fetoprotein) level is measured in the fluid. An excessive level may indicate a fetal defect. The main abnormalities associated with a high alpha-fetoprotein level are those caused by defective closure of the spinal column (neural tube defects), but it can also be associated with defects of the brain, kidney, closure of the abdominal wall, and, less commonly, defects in the intactness of the skin (skin diseases, etc.). In cases where the alpha-fetoprotein level in the amniotic fluid is high, the fluid is sent for further testing (acetylcholine esterase), and genetic counseling is offered to decide on further management and whether any additional tests (ultrasound, etc.) should be performed.

What are the limitations of testing amniotic fluid?

  • Examination of the chromosomes under the microscope is a relatively crude test, which is not sufficiently refined to identify small defects in the genetic sequence that can also cause genetic diseases. Most genetic diseases cannot be detected using chromosomal tests.

  • There are a few relatively rare conditions in which a duplication or deletion of a small region (a single band) within a certain chromosome is present, which is too small to be identified by microscope in the amniotic fluid cells. After birth, based on clinical symptoms, such a deletion (or duplication) may be suspected, and at this time it is easier to identify the problem using a more specific test.

  • There are a few relatively rare conditions where there is a combination of normal and abnormal fetal cells. The chromosomes in the abnormal cells contain an extra chromosome or a chromosome of abnormal structure. This condition of a combination of normal and abnormal cells in the same person is known as mosaicism In these cases, although examination of the amniotic fluid usually reveals the different types of cells, it could happen by chance that all the cells checked may be normal, the problem being detected only postnatally.

  • There are, of course, non-genetic causes of mental retardation or defects, and these cannot be tested for in the amniotic fluid. The risk of defects or mental retardation, even if the chromosomal result is normal, is 2%-3%.

  • There are rare cases in which examination of the chromosomes indicates one sex and the ultrasound scan indicates (externally) the opposite sex.

When is amniocentesis performed?

Amniocentesis is usually performed between the 16th and the 20th week of pregnancy, so that the result will be available by the 23rd week. Sometimes indications for amniocentesis only appear at a later stage of the pregnancy, and the test can be performed at this time, even in the third trimester, if there is a justifiable medical reason.

Time required for obtaining the result of a standard amniocentesis test

Usually, the first result can be obtained within 14-21 days, and a final result in 14-28 days. The amniotic fluid alpha-fetoprotein level can be determined within about a week.

What is done in cases in which an urgent result is needed?

There are a number of ways of obtaining a result quickly, and the genetic counselor should be consulted if necessary.

  • Chromosome testing using fluorescent staining (FISH - Fluorescent In-Situ Hybridization) may be used. This test enables the laboratory worker to locate defects in a number of chromosomes using a specific fluorescent "marker" for the chromosome examined. The test is thus somewhat limited with regard to the scope of information provided. On the other hand, if there is suspicion of a specific problem (for instance, Down syndrome, in which there is an extra number 21 chromosome), a reliable, rapid reply may be provided.

    As most chromosomal anomalies involve chromosomes 13, 18, 21, X and Y, the use of 5 separate colored fluorescent markers for these chromosomes, each chromosome having a different color, will give a reliable result for numerical defects in these chromosomes. This test is expensive, and is not usually funded by the health funds. In any case, the result of the standard chromosome test should be awaited (two weeks) in order to make sure that there are no anomalies in the numbers of the other chromosomes, so that the FISH test should only be performed if there is a specific indication.

  • The chorionic villi may be sampled even in advanced weeks, particularly in the case of a placenta previa.

  • A fetal blood test may be performed (umbilical cord puncture - cordocentesis).

Additional amniotic fluid tests

Apart from testing amniotic fluid for chromosomes and alpha-fetoprotein, additional tests can be performed for certain other specific conditions. These tests are usually performed following genetic counseling in cases where there is a high risk of a particular genetic disease and there is a reliable amniotic fluid test available to confirm or rule it out.

With advances in knowledge about the human genome, the number of special genetic tests will increase following the identification of more couples with a "tendency" for severe genetic diseases, as will the possibility of testing for these in the amniotic fluid. Today, most subjects tested for specific genetic diseases are couples who have already had a child with that disease, or for whom the screening tests available today have indicated the possibility of a genetic or infectious disease that can be tested for in amniotic fluid. Examples of such conditions are:
  • Intrauterine infection by pathogens such as CMV, Toxoplasma, etc.:
    This test is intended mainly for women in whom evidence has been found in preliminary blood tests or other tests (ultrasound, etc.) of a new infection during pregnancy involving these pathogens. In this case there is a risk of fetal infection leading to damage (defects, mental retardation, etc.). Amniotic fluid is then sent to a laboratory for specific testing for the pathogen concerned. The test is usually performed between the 20th and 22nd weeks of the pregnancy, the exact date being fixed specifically for each mother based on the estimated date of the onset of the illness. The fluid must reach the laboratory sterile and fresh. This means the test must be performed in the morning and the fluid transferred to the laboratory on the same day by special arrangement.

  • Enzymatic/biochemical tests:
    In amniotic fluid cell culture, the level of the activity of certain enzymes can be measured in the same way as in skin tests in adults. In cases where clinical/laboratory suspicion indicates that the fetus has a high risk of suffering from a specific enzyme deficiency (e.g. Tay Sachs disease, Gauchers disease and other metabolic disorders), the cultured cells can be sent to a laboratory that performs the specific enzymatic test.

  • DNA testing using molecular genetic techniques
    In pregnancies in which the couple has a genetic tendency for having a child with a genetic defect that can be tested for using molecular genetic methods, the culture fluid or cells can be sent to a laboratory that performs specific testing for the gene that causes that disease. Usually these are families with a specific genetic disease in whom tests demonstrating the mutation responsible for the disease have been performed and where there is a possibility to test for the disease in the amniotic fluid. The same applies to families who have undergone genetic screening tests (for CF, fragile X, etc.) and who have been found to have a high risk of having a child with one of these diseases.

Risks associated with amniocentesis

As noted earlier, the risk for injuring the fetus during the puncture is negligible. However, amniocentesis has a 1 in 200 to 1 in 300 risk of causing a miscarriage, which usually results from the onset of premature labor, breaking of the waters, bleeding, or infection of the amniotic fluid. This is only a problem in the two days immediately following the procedure. The risk to the mother's life is very low (1 out of 30,000 tests), and is associated mainly with infection of the amniotic sac.

Ominous signs following puncture:

  • Fever (over 37.7 degrees Celsius).

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

  • Breaking of waters or bleeding.

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

Who should undergo amniocentesis?

  • Women who have a higher than normal alpha-fetoprotein level in the second trimester biochemical screening test, because this can indicate an increased risk of neural tube defects.

  • Couples who have had a fetus with a chromosomal problem or neural tube defect in a previous pregnancy.

  • Couples where one partner carries a chromosomal defect such as a reciprocal translocation demonstrated in chromosomal tests.

  • Women who have an extremely low level of marker UE3 in the first trimester biochemical screening test without a family history of the skin disease ichthyosis.

  • Couples at increased risk of having a child with a genetic disease because one or both partners are known carriers of a specific gene (depending on the diseases inheritance pattern).

  • Suspected intrauterine infection with a virus or bacterium that may damage the fetus.

  • Abnormal findings in the ultrasound test performed on the fetus.

  • The mother's desire to perform the test even if none of the reasons stated above is relevant.

Importance of genetic counseling before amniocentesis

It is important to offer genetic counseling before amniocentesis in all cases where it is performed for medical reasons. Such counseling is intended to make sure that all the risks have been understood (the risks inherent to the procedure as well as those of finding genetic abnormalities in the test), and that there are no other special tests (of the amniotic fluid or blood, ultrasound, etc.) that should be performed in addition to the routine test. Special tests will be added if there is a medical reason based on the clinical data or family history or as a result of the findings of tests performed on the mother or her partner. Prior to counseling, the financing of the test by a health fund or the Ministry of Health should be determined.
 
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