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The mother's age
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What effect does the mother's age have?
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Any woman aged 35 or over at the date of her last period is at an increased risk
for having a child with numerical chromosome disorders. The most common disorder
is Down syndrome (trisomy 21). This risk is present at every age, but is significantly
increased with increasing maternal
age. For example, at the age of 20, the risk of having a child with Down
syndrome is 1:1923; by the age of 35 it has increased to 1:380, and at the age of
42 it is 1:67.
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Most HMOs recommend that women aged 35 or over undergo prenatal testing (chorionic
villus sampling or amniocentesis).
This test confirms or rules out the presence of a fetus with numerical or structural
chromosome disorders.
These disorders occur in the fetuses of older mothers because the mother's age affects,
in a manner that is as yet unknown, the process of meiosis (the reduction division
of the cell nucleus that leads to the formation of the ova).
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Is chorionic villus sampling or amniocentesis mandatory?
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In especially "precious" pregnancies, where the parents are afraid to put the pregnancy
at risk by undergoing invasive tests (chorionic
villus sampling or amniocentesis), biochemical screening and nuchal translucency
tests can be performed (see also the chapter "Identifying women at increased risk
of having an infant with Down syndrome"). These are screening tests and evaluate
the risk for Down syndrome at a relatively high predictive level. The mother's age
is taken into account in order to weigh the risk.
The advantage of the biochemical screening and nuchal translucency tests is that
they are non-invasive and therefore do not put the fetus at risk. Studies have shown
that the screening tests for predicting the statistical risk for Down syndrome are
more reliable in older women than in younger women. Therefore, in essence, an older
woman who receives a result interpreted as a low risk for Down syndrome in the fetus
is indeed at low risk.
The disadvantage is that these are screening rather than diagnostic tests. Furthermore,
they identify only Down syndrome and trisomy 18 and not disorders of other chromosomes.
Therefore it is not possible to eliminate, based on screening tests, the risk for
structural and numerical chromosome disorders. If a couple is considering not undergoing
amniocentesis, it is important for them to consult their physician and a genetic
counselor about the results of the screening tests and make the decision of whether
or not to undergo amniocentesis based on them.
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