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Auxiliary Invasive Tests
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Amniotic fluid test (amniocentesis)
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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?
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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.
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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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