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Why is there no medical recommendation for amniocentesis in all cases?
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Amniocentesis
(and chorionic
villus sampling) is undoubtedly the only test that can categorically rule
out chromosomal disorders. However, it is neither medically indicated nor recommended
that every pregnant woman should undergo amniocentesis - only those who are at high
risk of having a specific problem that can be examined by this test. Why?
The reason lies in the fact that pregnancy involves many risks, a great number of
which cannot be tested for and which therefore remain untested - this is a compromise
that we must accept at the present time.
Every pregnancy carries a 3%-4% risk of serious problems that are either detected
at birth or during the first year of life. These include mental retardation, autism (PDD ), metabolic disorders, deafness, muscular dystrophy, dwarfism, dystrophic diseases of the brain,
etc. Even after obtaining normal results for ultrasonography and amniocentesis,
the majority of these problems cannot be categorically ruled out. A small number
of these conditions can be tested for by using special techniques - these are recommended
for families in which such diseases are known to be present - i.e. families at high
risk.
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The cost of each test that can currently be performed is enormous, so that these
tests are not recommended in all cases.
Because we currently have to compromise on a considerable percentage of genetic
problems, the risk in some of these conditions that are not tested for routinely
can, for instance, be 1:2000 (i.e. out of 2000 mothers with similar characteristics
undergoing amniocentesis, only one will be found with this syndrome).
Similarly, there are no medical grounds for checking specifically for Down syndrome or
trisomy 18 if the risk is 1:2000.
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Of course, financial cost-benefit calculations and the risk of miscarriage as a
result of amniocentesis are taken into account.
The medical indications for undergoing amniocentesis are similar throughout the
western world. Twenty years from now, it may well be possible to check for the entire
spectrum of diseases with high reliability and at a relatively low price.
Notwithstanding this, the current feeling is that amniocentesis for chromosome analysis
is important to the higher socioeconomic and non-religious classes in the western
world. Twenty-four percent of all pregnancies in these classes undergo amniocentesis,
and a large proportion of these are undertaken privately. The test also allows for
the detection of less severe chromosomal problems (from the point of view of expression
or their risk of mental retardation) than Down syndrome, such as sex chromosome
disorders, translocations, and so on - the average risk for these problems in each
pregnancy is 1:400. This risk further increases with maternal age, reaching 1:100 over the age of 35.
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