What is amniocentesis?
An
amniocentesis involves taking a small sample of amniotic fluid (water)
from around the developing fetus. A needle is first passed through the
pregnant woman's skin, then through the wall of the uterus (womb), and on
the amniotic fluid. Approximately 15 mls (3 teaspoonsful) of fluid is
taken, this amount being small compared with the total amount of fluid
present at the time of the test at 15 weeks.
Genetic Counselling
Genetic Counselling maybe offered by your own doctor or alternatively
is available at the practice prior to C.V.S. and other procedures. This is
to help you understand both the test itself, plus birth defects and
inherited conditions in general; it also allows relevant family medical
and pregnancy information to be checked to assess your specific risks. You
will then be informed of the risks, benefits and limitations of testing to
you. You can choose the course of action that is appropriate to you in
view of your own specific family needs and goals.
We recommend counselling on a day prior to the procedure to enable you
to better consider your options, but it is available also at the practice
on the day of the procedure.
Who is offered amniocentesis?
Women of 37 years and over at the estimated time of delivery are
routinely offered testing in Victoria. Some younger women may also be
offered it (in many other states of Australia and other countries it is
offered at 35 years and sometimes even younger).
Other reasons for offering an amniocentesis include:
- Women who already have a child with chromosomal problem such as Down
Syndrome,
- Women who know that an inherited disorder runs in their family or in
the family of their partner.
- Women who have an abnormal 'serum screen' test - a blood test for
determining who is at increased risk of a fetus with Down Syndrome or
spina bifida,
- Women who have a child with a neural tube defect such as a spina
bifida
- Occasionally amniocentesis is performed for other reasons. These
include couples who are known to have a chromosome abnormality
themselves, plus those known to be at risk of having a baby with one of
a number of other rare disorders.
Why are older women offered amniocentesis?
As the woman's age increases her chance of having a baby with a
chromosome abnormality, such as Down Syndrome, also increases. The chance
of Down Syndrome is the same whether or not this the first child.
In each human cell there are 46 chromosomes, each of which is paired
giving 23 different chromosome pairs. The only exceptions are the egg and
sperm which each have 23 chromosomes. Each chromosome contains thousands
of genes which control an individual's growth and development. Birth
defects may result if there are too many or too few chromosomes. The best
known chromosome abnormality in newborn babies is an extra chromosome 21
so that the baby has 47 chromosomes in the cells instead of the normal 46.
This causes a condition known as Down Syndrome (previously known as
mongolism). A chromosome abnormality cannot be cured or corrected.
The chance of having a live born baby with Down Syndrome when you are
aged 37 years is approximately 1 in 220. The chance at 40 years is
approximately 1 in 90. The chance of having a live born baby with any
chromosome abnormality is about double these figures. With increasing age
there is no increase in the chance of having a baby with spina bifida.
Most other abnormalities also do not depend on the age of the pregnant
woman so that if the chromosomes are normal then most of the increased
risk in 'older' pregnant woman has of delivering a baby with an
abnormality is removed.
It is important when looking at the chance of a baby having a
chromosome abnormality to keep the figures in perspective:
- approximately 4% of babies have some abnormality at birth and most
of these have normal chromosomes.
- chromosome abnormalities are rare compared to many of the other
risks of pregnancy, for example 5% of babies are born early.
Amniocentesis: ultrasound is used to guide the
needle into a pocket of fluid |
What type of disorders can be detected by amniocentesis?
The abnormalities looked for include:
- Chromosome abnormalities such as Down Syndrome
- Neural tube defects
(ie spina bifida or anencephaly),
- Some inherited disorders in those couples who have been shown to be
at high risk, eg cystic fibrosis.
Amniocentesis is not a general test for the fetus - it tests for only
very particular problems, usually only chromosomal abnormalities and spina
bifida.
When is amniocentesis carried out?
We carry out amniocentesis at approximately 15 weeks. It is probably
safer to wait until this time although 'early amniocentesis' is offered at
some centres prior to 14 weeks.
Why is an ultrasound examination needed before the
amniocentesis?
An ultrasound examination will first be carried out to confirm the age
of the pregnancy, and to make sure there are not twins, and to check for
physical abnormalities of the fetus.
By locating the position of the fetus and placenta, steps can be taken
to avoid them during the amniocentesis. Ultrasound is used throughout the
amniocentesis procedure to first guide the needle to the safest position,
and then to keep the needle away from the fetus.
Your doctor may arrange for another scan at around 18 to 20 weeks when
a more detailed check is possible.
What should I do before the Test?
It is not necessary to have a full bladder, however having some urine
in the bladder may make it easier to examine the fetus and lower part of
the uterus.
You should expect to be at our practice for approximately 1.5
hours.
What happens on the day of the test?
When the scan has been completed the amniocentesis is performed without
you having to move from the examination couch.
After cleaning the skin, a very fine needle is inserted into the uterus
watching the needle all the time on the ultrasound screen. It then only
takes about 30 seconds to draw the required 15 mls (3 teaspoons) of fluid
into the syringe. The needle is removed and the fetus is again checked.
After waiting for about 20 minutes you are then able to return home.
Patients usually feel far less discomfort from the test than they
expected - many indicate that it is not even as bad as a simple blood
test. If you are worried about the test nitrous oxide, the gas used in
labour, is available, although most patients find they do not need it.
What is done with the cells?
Amniotic fluid contains cells which have been shed by the
fetus. They
are allowed to grow in the laboratory until there are enough to test the
chromosomes. Each of the chromosomes from a number of cells is carefully
examined to check that the correct number of cells is carefully examined
to check that the correct number are present ad that the appearance and
length of each chromosome is normal. If the fetus has Down Syndrome
(mongolism) then there is an additional chromosome 21 present. The sex can
also be determined by looking at the chromosomes - you can find this out
if you ask your doctor.
The laboratory will usually have the chromosome result available in 2
weeks - your doctor will be informed and he or she will pass it onto you.
The results of the test for spina bifida will be available at the same
time.
How will I feel afterwards?
While you will be quite capable of driving yourself home after the
test, most people appreciate having a companion to drive home. It is
recommended that you rest for the remainder of the day although there is
no reason for you to go to bed. There need be no further limitation on
your normal routine after this time.
If you have to travel more than one to two hours to get home, it is
preferred that you stay in Melbourne overnight afterwards.
Occasionally after the test patients may have either mild discomfort or
some bruising under the skin but neither causes any problem to the
pregnancy.
What complications can occur?
In a study Lachlan de Crespigny carried out, including his one other
doctor's results, the miscarriage result was not demonstrably higher than
those who did not have the test performed. The risk of the test causing
miscarriage therefore is very low, a best estimate is 1 in 200 or less.
There is also about a 1 in 100 chance of having some leakage of amniotic
fluid through the vagina for one to two days after the procedure. This
usually stops if you rest and it can be expected to cause no problem to
the fetus or the pregnancy.
If your blood group is Rh negative, anti-D is usually given only if the
needle passed through the placenta. Direct injury to the fetus by the
needle has not been reported after any of our patients. Rarely have any
complications to the pregnant woman or other complications to the fetus
been reported.
Couples often ask when a miscarriage would occur. This is a difficult
question because at least two out of three miscarriages after
amniocentesis are totally unrelated to the test itself and would have
occurred anyway. However, if you have had no problem by three weeks then
any complications is very much related to the experience and expertise of
the operator.
The test, like any other, may fail because no specimen is obtained or
because the laboratory are unable to grow cells. Both of these are very
uncommon.
Incorrect results are extremely rare. Occasionally uncertain results
may occur - these may need a further test such as a sample of fetal blood
to sort them out.
What should I look for after the test?
If you lose blood or clear fluid from the vagina after the test then
you should contact your own doctor.
What if the amniocentesis shows that an abnormality is
present?
Those few patients in whom the tests show that the fetus has an
abnormality will be given information so that they can make a choice about
whether to continue the pregnancy. It is still possible to perform a
termination of the pregnancy at this stage. Counselling is available to
help couples make their decision.
Should I have an ultrasound examination later in the
pregnancy?
This depends on how well the fetus can be seen at the time of the
amniocentesis. Usually details of the structure are difficult to see at
this early stage, so your doctor may recommend a repeat examination
later.
What does the test cost?
Details of charges are available at our practice. Please note that the
laboratory will send a separate account for the analysis of the specimen
plus any necessary blood test. Medicare will cover part of the cost of
these accounts.
Summary
In experienced hands, amniocentesis is a straight forward procedure
which is well tolerated by patients with special indications, the
commonest being the age of the pregnant woman. The test checks for Down
Syndrome and other chromosome abnormalities, and also for spina bifida. It
is important to realise that while the combination of the ultrasound scan
and amniocentesis goes a long way to ensuring that the fetus is normal, it
does not guarantee that every possible abnormality has been excluded.
If you would like more information, please ask. If you would like more
written information we have pamphlets available on ultrasound and other
procedures. For more detailed information, you may purchase a book Lachlan
has written, "Which Test For My Unborn Baby? A Guide to Prenatal
Diagnosis" (OUP) which is available at a reduced price at our
practice.
| 
