Chorionic Villus
Sampling
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What is the
test?
Chorionic villi are small structures in the
placenta that act like blood vessels. These structures
contain cells from the developing fetus. A test
that removes a sample of these cells through
a needle is called chorionic villus sampling
(CVS).
CVS answers many of the same questions as amniocentesis
about diseases that the baby might have. Diseases
that can be diagnosed with CVS include Tay-Sachs,
sickle cell anemia, cystic fibrosis, thalassemia,
and Down syndrome. (Rh incompatibility and neural
tube defects, however, can be diagnosed only
through amniocentesis.) CVS can be done earlier
in pregnancy than amniocentesis and can be done
when there is not enough amniotic fluid to allow
amniocentesis. However, it has some extra risks
when compared with amniocentesis.
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How
do I prepare for the test?
CVS can be done between the 10th and 13th weeks
of pregnancy. Tell your doctor ahead of time
if you have ever had an allergic reaction to
lidocaine or the numbing medicine used at the
dentist's office.
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What
happens when the test is performed?
There are two ways that your doctor can perform
CVS. Some patients have the sampling done through
the vagina and cervix. Most patients have the
sampling done through the abdominal wall. For
both types of sampling, you lie on your back
on an examination table and the doctor uses ultrasound
to locate the fetus and the placenta.
If the sampling is to be done through the vagina
and cervix, you place your feet in footrests
and bend your knees up, as you would for a pelvic
examination. A speculum (a device that looks
like a duck-bill that can be opened and closed)
is used to open the vagina so that your doctor
can see inside. A long tube, much narrower than
a straw, is inserted through the cervix and moved
forward while your doctor watches on the ultrasound
until it is next to the fetal side of the placenta.
A small sample of the lining around the fetus
is then pulled into the tube for testing.
If the sampling is to be done through the abdominal
wall, your lower abdomen is cleaned with an antibacterial
soap. In some cases, the doctor uses a small
needle to inject a numbing medicine just under
the skin, so that you do not feel the sampling
needle. (Because the sampling needle does not
cause much more stinging than the numbing medicine
itself, not every doctor includes this step.)
A hollow needle several inches long is inserted
through the skin and muscle of the abdomen and
through the wall of the uterus, to the edge of
the placenta. This needle is held in place as
a guide needle. A narrower needle is then inserted
through the first needle and is rotated and moved
inward and outward a number of times while a
sample is collected into an attached syringe.
The fetal heart tones and the mother's blood
pressure and heart rate are checked at the beginning
and end of the procedure. The whole procedure
takes close to 30 minutes.
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What
risks are there from the test?
The risk of miscarriage and other complications
from CVS is slightly higher than the risk from
amniocentesis, although some parents feel that
it is worth the extra risk to be able to makedecisions
earlier in the pregnancy if the results show
the baby has a health problem. There have also
been some reports that suggest there is a very
small risk of birth defects (abnormal limbs)
in the fetus.
One particular difficulty with this test is
that due to variability in the cells of the placenta
(called mosaicism), occasionally you can have
an abnormal test result even if the baby is normal
and healthy. This might lead you to make decisions
about pregnancy termination that you would not
have made if you had better information.
Some women have vaginal bleeding after the procedure.
Infection is uncommon.
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Must
I do anything special after the test is over?
You should notify your doctor if you are having
vaginal bleeding, especially if it is heavy or
lasts longer than three days. You should also
tell your doctor about strong abdominal pain
or fever.
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How
long is it before the result of the test is
known?
Chromosome analysis of the sample takes two
weeks or more. The results of some tests may
be available sooner.
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