Chorion Villus Sampling
What is CVS?
Chorionic Villus sampling is a test which involves obtaining
a small sample of tissue, the chorionic villi from the developing
placenta. The placenta which has the same chromosome makeup
as the baby may be then examined to check for genetic, DNA or
biochemical abnormalities eg Down Syndrome, Cystic Fibrosis
etc.
Who is offered CVS?
Most testing by CVS is offered to patients who are at particular
risk of chromosomal abnormality such as Down syndrome. This
would include mothers who are 37 years or older, and those
who have had a previous Down syndrome baby. The test is also
offered to those who have had a previous baby with other chromosomal
or genetic abnormalities (such as metabolic disorders). On
occasion the test is performed for those who are particularly
anxious regarding the risk of chromosomal abnormality.
How is the test Performed?
The skin of the lower abdominal wall is cleansed with an antiseptic
alcohol based solution. The skin and underlying tissues are
injected with local anaesthetic. The discomfort felt during
the injection of the local anaesthetic is similar to that
felt when blood is drawn from the forearm. With ultrasound
control a fine needle is then guided into the placenta and
a biopsy of placenta tissue is taken (chorionic villi). Sometimes
a dragging sensation is felt in the pelvis or in the legs
during the procedure. This is a normal feature of the test.
Also sometimes more than one biopsy is required.
When is the test Performed?
Ideally the test is performed between 11-12 weeks gestation
, however it may be performed as early as 91/2 weeks and as
late as 19 weeks.
What preparation do I need before
the Test?
A moderately full bladder is preferable. This brings the pregnancy
up into the abdominal cavity and thus accessible for the needle
test. If you begin to feel very uncomfortable with a distended
bladder then try to let some urine out as this may also hinder
the test.
A scan is performed before the test in all patients to:
- Make sure the baby is alive and well,
- To determine whether there are twins or higher multiples
- To check for abnormalities in the baby including the nuchal
translucency (ref.10-14 week sac information leaflet). A
variety of abnormalities may be detected at this early stage
of pregnancy.
Finally it is also very important to know what your blood
group is before the test, (ie Rh positive or negative). If
possible ask your doctor to provide this information or bring
along you blood group card at the time of the test. If you
are negative blood group you will need an injection of anti-D
after the test. This may be explained further by the Doctor
at the time of the test.
What happens the biopsy after it
as been taken?
The specimen is sent to the laboratory and processed. The
tissue is placed in a culture medium and then into an incubator
for several days. When there are sufficient number of dividing
cells the specimen is removed from the incubator and the placental
cells are split open with an enzyme. The individual chromosomes
are counted and analysed.
Every cell should contain 23 pairs of chromosomes. The test
looks for an extra chromosome number 21 (Down syndrome). Each
of the other 22 pairs of chromosomes are also examined. This
includes chromosome pairs number 1,2,3,4, and so on up to
the sex chromosomes (pair no 23). Therefore this test not
only excludes an extra chromosome number 21(Down syndrome)
but it excludes extra chromosomes in the other 22 chromosome
pairs.
Also each individual chromosome pair is then stained with
a special dye and examined under ultraviolet light. The individual
bands of each chromosome are examined in great detail for
subtle genetic abnormalities such as 1)insertion of genetic
material into a chromosome , 2)deletion of genetic material
from a chromosome and 3) exchanging of genetic material between
chromosomes (called translocations). So this test excludes
not only Down syndrome but a wide variety of additional subtle
and major chromosome abnormalities.
What should I do after the test?
It is recommended that you take rest for the remainder of
the day. This does not mean you should confine yourself to
bed but rather you should just rest at home and avoid any
strenuous activity including lifting any heavy weights. Most
patients experience a short duration of mild crampy period-like
pains, lower abdominal pains or even leg pains after the test.
This is most likely to occur after the local anaesthetic wears
off, ie within the first half hour after the test. Some patients
experience slight vaginal blood spotting after the test which
is also not unusual.
What are the risks of the test?
There is a 1% risk of miscarriage with the test. This usually
related to infection introduced at the time of the procedure.
Several antiseptic precautions are taken to minimise this
risk.
The needle itself is at all times well distanced from the
baby. In fact when the biopsy is taken the needle is positioned
outside the pregnancy sac (ref. diagram overleaf).
Warning signs of miscarriage include strong regular period
like pains with fresh red bleeding. Call your doctor should
this occur. The time when miscarriage is most likely to occur
is the first 24-48 hrs-7days after the test. Additional Complications
of the test include failure of the specimen to grow sufficiently
in the laboratory and uncertain laboratory results. These
complications are very uncommon and if they do occur you still
have the choice of an amniocentesis test at 15-16 week of
pregnancy. This test will give the same information about
the baby's chromosomes (refer amniocentesis information leaflet).
Diagramatic representation of a CVS
The mother's head is to the left and the legs to the right.
The needle is directed under ultrasound control into the developing
placenta. One or sometimes two biopsies are taken.
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