AMNIOCENTESIS & TRIPLE TEST
Amniocentesis
An amniocentesis is a test in which a small sample of fluid from the womb is taken . It involves passing a needle through the skin of the
mother, through the wall of the womb and on to the fluid without touching the
baby. Approximately 20 ml of the fluid is taken. This amount is small compared
to the amount of fluid in the womb. It is done under the guidance of
ultrasound.
Laboratory tests are carried out.
A test is carried out on the cells which are present in the fluid. It involves
culture of the cells and study the chromosomes from the nucleus of the cells.
It is possible to find out if the baby has
Who should have the test?
The reason for
advising women over 35 years of age to have amniocentesis is as the age of the
women increases, the chance of her having a baby with a chromosomal
abnormality also increases. The commonest chromosome abnormality is Down?s syndrome. It does not
matter if the women had previous normal deliveries. The chance is there with
advancing age.
|
Chromosomal abnormality |
|
|
Maternal age |
% chromosomal abnormality |
|
35 |
1.25 |
|
36 |
1.41 |
|
37 |
1.50 |
|
38 |
1.65 |
|
39 |
2.10 |
|
40 |
2.36 |
|
42 |
4.01 |
|
45 |
7.28 |
When to carry out amniocentesis?
It is considered safer for the fetus to wait till at
least 16 weeks when the pregnancy is stable and the fluid is adequate for the
sampling. The procedure can be done in the clinic. It not much discomfort and the procedure takes about less than a minute
to insert the needle and aspirate the fluid from the womb. Most patient inform us that they experienced far less discomfort
from the test than they had expected. Any discomfort that is felt is usually
very minor. After a short rest of half an hour, patient can return home. It is
advisable to take it quietly for the rest of the day and possibly the next day
although there is no reason to stay in bed all the time.
Complications
The complication of miscarriage quoted for the procedure
is 0.5%. Apart from the very slight risk of a miscarriage 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 either because no
specimen is obtained from the sampling or because the laboratory cannot produce
a result. However, this situation is very rare.
I usually ask the patient to return in a week for a check up. Patients were
told to return earlier should there be any loss of blood or water from the
vagina or any other pains after the amniocentesis.
Results
The chromosome test for Down?s
syndrome takes 2 to 3 weeks because of the requirement to grow the cells in the
laboratory. The laboratory will send the results directly to the clinic. Recently
there is improvement in the examination technique that the results may be
obtained at an earlier date.
When the result showed that there is abnormality, it is still possible to
perform a termination of the pregnancy before 24 weeks.
Triple test
For patients who do not wish to have an amniocentesis
or patients with ages less than 35, the next best is to do a blood test called
the triple test. The test involves taking 5 ml of blood to measure 3 hormones
– fetoprotein (AFP), gonadotropin (HCG) and unconjugated estriol (UE3). The
results with the age of the mother will be computed to give a risk factor. It
is a screening test and not actual study of cells like in amniocentesis. It
cannot pinpoint which fetus is actually affected.
Women with abnormal results indicated by higher risk than normal are advised to
have further testing. On the other hand, an abnormal result does not mean that
the baby definitely has a birth defect. It is a prenatal statistic risk
assessment only.
Even though the triple test suggests that the risk is low, it cannot guarantee
a normal baby or be certain that everything is all right with the baby. It is a
screening test and will detect 85% of all open neural tube defects, about 75%
of ventral wall defects, and approximately 60% of babies affected with Down
syndrome. There are other kinds of problems the Triple Test cannot detect.
The best time for the Triple Test to be performed is between the 15th and 18th
week since the last menstrual period. The result can be known in a couple of
days.
Being a prenatal risk assessment, the decision of accepting the risk or not is
very much dependent on the patient.
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[Copyright 2005] |