What is Amniocentesis?
This is a test which may be offered to a woman whose
age, family or medical history, nuchal or blood test
results suggest that she may have a higher than normal
risk of having a baby with a specific
abnormality. This pamphlet gives you some information
about amniocentesis. You and your doctor will decide
whether it is appropriate for you to have this test.
Who may be offered Amniocentesis?
1. Women in whom an ultrasound scan or blood test
has suggested that the fetus is at increased risk of
having a chromosomal abnormality such as
Down Syndrome.
2. Women of 35-37 years (depending on the State)
and over at the estimated time of delivery.
3. Women who have already had a child with a
problem such as Down syndrome or spina bifida.
4. Occasionally amniocentesis is done for other
reasons. These include parents who are known to
have a chromosomal abnormality themselves or
who are know to be at risk of having a baby with
one or a number of rare “metabolic” disorders.
Why does age matter?
As the age of a mother increases, the chance of her
having a baby with a chromosomal abnormality also
increases. The commonest chromosome abnormality
is Down Syndrome. Between 35 and 40 years of age,
the chance of having a baby with a chromosomal
abnormality is approximately 1 in 150. Beyond the
age of 40 years this chance increases even further.
This does not apply to spina bifida where the chance
of having a baby with this condition does not increase
with the increasing age of the mother.
How and when is the test done?
While an amniocentesis can be performed at any stage
in pregnancy, it is usually done at around 15 weeks.
The mother will have an ultrasound examination, which
will confirm the dates of her pregnancy and show the
position of the baby and placenta. Using a slender
needle which is guided by ultrasound, the doctor
withdraws a small amount of amniotic fluid
(about one tablespoon) from around the developing
baby. Normally this fluid is used to carry out two tests.
1. The amount of a particular protein (alpha fetoprotein)
is measured. There is usually an increased
amount of this protein present in the fluid when the
baby has the abnormality called spina bifida.
2. A test is carried out on the cells which are present
in the fluid. Using special procedures in the
laboratory, the chromosomes in these cells can be
examined and in this way it possible to find out if
the baby has Down syndrome, or any of the other
known less common chromosome problems.
Also, by looking at the chromosomes, the sex of the
baby can be determined.
What happens the day of the test?
It is not necessary for the mother’s bladder to be full.
However, having a little urine in the bladder may make it
easier to see the baby and the lower part of the uterus.
When the ultrasound scan has been completed, the
amniocentesis itself is performed without the mother
having to move from the examination table.
After cleaning the skin, a needle is inserted into the
womb, guided all the time by the ultrasound image.
It then takes only about thirty seconds to draw up the
tablespoon of fluid needed into a syringe.
After the needle is removed and the baby checked
again, the mother may leave the room immediately to
empty her bladder and dress.
After sitting quietly for 20 minutes or so, she can go
home. Only disposable (once-only) needles are used.
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Any discomfort felt is usually minor. Almost without
exception, women experience far less discomfort from
the test than they had expected.While most women are
quite capable of driving themselves home after the test,
it is suggested that, where possible, a companion
should drive.
It is normally recommended that the woman should
take things quietly for the rest of the day although there
should be no reason to go to bed. After this time, she
may return to her normal routine.
What complications can occur?
The risk of the test causing a miscarriage is low, not more
than 1 in 200. It should also be noted that even if an
amniocentesis is not performed there is still a risk of up to
1% of a spontaneous miscarriage.
In about 0.5% of patients, there is some leakage of the
amniotic fluid through the vagina in the 24-48 hours after
the procedure. In almost all cases this stops on its own
and does not cause any problems to the baby or to
the pregnancy.
The amniocentesis test does not appear to be
associated with any other complications to the baby or
to the pregnancy.
The test, like any other, may fail to give a result, the most
common reason being insufficient material for the
laboratory to examine. This is very uncommon.
how long will the results take?
The result of the test for spina bifida is usually available in
a few days. The chromosome test for Down syndrome
takes 10-14 days because the laboratory has to grow
the cells. A rapid test called fast FISH is also available.
The results come back in 1-2 days. We ring you with the
results or you can come in to see us.
Summary
Amniocentesis is well tolerated by patients and has
very few complications. It is, however, only performed
in patients with special indication, the most common
being because the mother is aged 35 years or more.
The test checks for Down syndrome, other chromosome
abnormalities and also spina bifida.
It is important to realise that while the combination of the
ultrasound scan and amniocentesis goes a long way in
checking that the baby is normal, it does not guarantee
that every possible abnormality has been excluded.
Dr Philippa Ramsay MBBS FRANZCOG DDU COGU
Dr Linda Atkins MBBS FRANZCOG DDU
Dr Joanne Ludlow MBChB FRANZCOG FRCOG DDU
Dr Karen Mizia MBBS BSc FRANZCOG DDU
Dr Indika Alahakoon MBBS FRANZCOG DDU CMFM
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