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A rise in older women becoming pregnant has led to a big jump in
Down's syndrome diagnoses in Britain, but fewer babies are born
with the condition because more than 90% are aborted, researchers
said.
Scientists found that diagnoses of Down's syndrome rose by 71%
between 1989 and 2008, largely due to a sharp rise in the number of
women wanting to become mothers later.
But the number of babies born with Down's fell by one percent
because more women were screened and chose to terminate the
pregnancy.
"Dramatic changes in demography have been offset by improved
medical technology and have resulted in no substantial changes in
the birth prevalence of this quite disabling condition," the
researchers wrote in the study in the British Medical
Journal.
The risk of having a baby with Down's syndrome - which occurs when
a child has three copies of chromosome 21 instead of the normal two
- increases sharply as women get older.
The risk for a 40-year-old mother is 16 times that for one who
is 25.
The researchers from Barts hospital in London and The London
Medical School said that without antenatal screening and subsequent
abortions, the number of Down's syndrome births would have
increased by 48% between 1989 and 2008 due to parents choosing to
start families later.
People with Down's often have common physical features such as
almond eyes and shorter limbs, and have learning difficulties as
well as a higher risk of congenital heart defects and respiratory
illnesses.
The syndrome is usually detected by tests at around 11 weeks of
pregnancy involving first an ultrasound to measure the thickness of
fluid in the baby's neck and then taking blood from the mother to
test for proteins that suggest abnormalities.
The study showed that around nine out of 10 women (92%) with an
antenatal diagnosis of Down's syndrome decide to terminate the
pregnancy - a figure that has remained constant despite the more
than 70% rise in diagnoses.
It also showed that while there has been a stark increase in the
proportion of women under 37 opting for Down's screening (from
three percent to 43%), the proportion of older women having a test
had stayed constant at around 70%.
As more women are having children later in life and a significant
proportion of these mothers are deciding against screening, a large
number of births with Down's syndrome are still likely, the
researchers said, adding that monitoring numbers of babies born
with Down's syndrome was essential to ensure adequate provision for
their needs.
In 2001, British health authorities advised that all pregnant
mothers should be offered a test for Down's.
The researchers said their study showed a need to find out why 30% of older pregnant women still decide not to be tested.
