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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 35yrs 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 determine 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 an
antiseptic. 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 colored 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 1/500 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.
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