Amniocentesis
1. What is Amniocentesis?
Amniocentesis is a procedure which involves passing a fine needle
through the maternal abdomen and the uterine wall into the amniotic
fluid around the fetus in order to obtain a sample of the amniotic
fluid. From this fluid, fetal cells are harvested, cultured
and treated to reveal their chromosomes. While an amniocentesis
can be performed at any stage in pregnancy, it is usually performed
between 14-16 weeks. The volume of fluid aspirated is about
1/6th of that present around the fetus and this is naturally
replaced over the next 24 hours.
2. Who may be offered Amniocentesis?
Amniocentesis is generally offered to patients who a) are
37yrs and over at the estimated date of delivery, b) to women
who have previously had a child with a chromosomal abnormality,
c) patients who themselves have a chromosomal abnormality
or other rare metabolic or genetic abnormalities and d) occasionally
to mothers who are extremely anxious regarding the possibility
of a chromosomal abnormality in their baby.
3. How is it performed?
An ultrasound examination is first performed to a) confirm
the dates, b) to assess the position of the placenta (afterbirth),
and c) assess the baby for ultrasound signs of chromosomal
abnormality such as Down Syndrome. A point on the abdominal
wall is selected by the operator using ultrasound. It is then
cleaned with antiseptic and local anaesthetic is injected.
The amniocentesis needle is then guided into the amniotic
fluid (the fluid surrounding the baby) by tracking it's course
on the ultrasound screen. It takes about 30 seconds to draw
up the straw coloured fluid.
4. What are the risks of the test?
The principle hazard of the procedure is the risk of introducing
infection into the pregnancy which may result in miscarriage.
This complication occurs in only 1/200 tests performed. The
warning signs of miscarriage include regular crampy period-like
pains with fresh red bleeding which occur in the first 24-48
hours after the test. A small fluid leak is a rare problem
with much less likelihood of progressing to pregnancy loss,
this usually settles of its own accord. Because the procedure
is performed with ultrasound guidance, fetal injury with the
needle is extremely rare.
5. What happens to the specimen taken?
A 15-22ml sample of fluid is aspirated and dispatched to the
laboratory. The baby's cells are extracted from the fluid
and placed in a culture medium in a warm incubator. In 7-10
days when sufficient cells have grown, they are treated so
the chromosomes are revealed. The chromosomes are examined
under the microscope with ultraviolet light. Each individual
chromosome pair (of which there are 23) are examined in detail.
The laboratory scientist will examine the cells for an extra
chromosome no. 21 (Down syndrome) and for any abnormality
in the other 22 pairs of chromosomes.
6. What is to be expected after the
test?
Most patients will experience a few crampy pains or some mild
crampy abdominal discomfort after the test. This is due to
irritation of the uterus by the amniocentesis needle or local
anaesthetic. It is safe to take paracetamol or panadol but
no stronger analgesic should be necessary. If the pains worsen
then contact your doctor. Some spotting after the test is
also not unusual, but should this progress to fresh red bleeding
then contact your doctor. Many patients may be fortunate and
experience no symptoms at all after the test. Take it easy
for the rest of the day although there is no need for bedrest.
It is probably a good idea for someone to take you home after
the test. You may carry on as usual the day after the test.
7. How long before I know the results?
Depending on the rate of cell growth in the incubator results
are available from as early as 10 up to 21 days. You will
be contacted by phone with the result and a written report
will also be mailed directly from the laboratory to your doctor.
It is your doctor who will tell you the sex of your baby,
if you want to know (this part of the result will not be given
over the phone).
8. Are the results accurate?
A chromosome result is one of the most reliable medical tests
which has an accuracy in the order of 99.9%.
What do I do following the procedure?
It is recommended that you have someone with you to drive
you home. You should rest for approximately 48 hours after
the procedure as you may experience some mild period type
abdominal pains. For any other problems you should communicate
with your own doctor.
Diagrammatic representation of an
amniocentesis.
The patient's head is to the left and the legs to the right.
The needle is advanced into the largest pocket of fluid
and 15-22 ml aspirated. It takes 5-10 minutes to set up the
procedure and 1-2 minutes to aspirate the fluid. Sometimes
the fetus may move in the way of the needle but this does
not it cause any harm.
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