| Until now, prenatal diagnosis of Downs
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| | median age was 34. Eight of the mothers
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| syndrome (Trisomy 21) for the unborn baby
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| | had their blood samples taken in the
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| has required an invasive test. This has
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| | first trimester (13 weeks or below).
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| taken the form of an amniocentesis where
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| | The breakthrough was that they managed to
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| fluid is taken from around the fetus in
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| | successfully identify and separate the
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| the womb after 14 weeks of gestation. The
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| | small amount of fetal DNA that cross into
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| alternative, available about 3 weeks
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| | the maternal circulation. This was
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| earlier is CVS where a tiny piece of the
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| | analyzed via what are known as SNP
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| afterbirth (placenta) is extracted.
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| | (single nucleotide polymorphisms) tests
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| Screening Tests
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| | to identify where there was an extra
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| The screening tests which are
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| | chromosome on set No. 21 (Trisomy 21).
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| non-invasive have varying degrees of
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| | Their tests in this study correctly
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| detection rates from a lowly 30% for
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| | identified 58 of the 60 chromosomal make
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| maternal age alone to a clearly
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| | up. Among the 58 correctly analyzed, two
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| impressive 85% for a combination of
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| | had Trisomy 21 (Downs). The two that were
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| maternal age, fetal nuchal translucency
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| | not correctly analyzed included one who
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| (scan) and maternal serum biochemistry at
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| | had Downs but labeled as normal (false
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| 11-14 weeks. This is called the
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| | negative) and the other one was
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| Integrated Test.
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| | incorrectly identified to have Downs
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| Research has also shown that fetuses
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| | syndrome (false positive).
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| affected by Downs syndrome have a missing
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| | What does this imply?
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| nasal bone on an ultrasound scan
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| | This is potentially quite a major
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| performed at 11-14 weeks. This test alone
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| | advance. Its strength lies in the fact
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| has been shown to identify around 70% of
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| | that it does away with an invasive
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| Downs syndrome babies. Crucially however,
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| | procedure therefore eliminating in a
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| it did not appear to give any advantage
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| | stroke the spectre of miscarriage that
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| over the longer established nuchal
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| | hangs over the currently used procedures.
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| translucency scan performed at the same
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| | This risk is indeed small but quite real
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| gestation and therefore the uptake has
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| | and when it occurs, is devastating.
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| been poor.
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| | However, it is important to stress that
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| One of the most frustrating weaknesses of
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| | this is only an early stage of this
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| all screening tests is the fact that they
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| | potential breakthrough. Here are the
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| also falsely detect unaffected babies,
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| | facts:
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| occasionally leading to unnecessary
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| | The study had quite a small
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| invasive diagnostic tests and always
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| | sample (60) and it is therefore crucial
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| causing significant stress to prospective
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| | that it is replicated in larger studies.
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| parents. The false positive rate of
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| | There is still a lot of room for
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| screening tests is around 5%.
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| | refinement. In this study, whilst the
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| Moreover, even the most sensitive
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| | earliest sample was taken at 8 weeks
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| screening test will still miss some
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| | gestation, the first case of Downs was
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| babies that are affected and for all
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| | identified at just over 17 weeks. For
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| parents this means a little seed of doubt
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| | this to be truly useful, it will have to
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| until the baby arrives.
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| | have the ability to establish the
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| Diagnostic Tests
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| | diagnosis early, ideally in the first
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| The definitive diagnostic tests mentioned
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| | trimester. Also the interval between
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| (amniocentesis and CVS) suffer from one
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| | blood sample collection and getting the
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| Achilles heel: They are invasive and
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| | results need to be reasonably short,
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| therefore carry the potential risk of
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| | lasting no more than 2 or 3 weeks.
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| provoking miscarriage. This can happen
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| | The current diagnostic tests
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| regardless of whether the baby is
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| | have quite a high 99% accuracy rate. This
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| affected by Downs syndrome or not.
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| | test will need to match that and ideally
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| Because of this, there has been a lot of
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| | surpass it.
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| research activity to find a risk-free
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| | Science research thrives on this kind of
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| reliable diagnostic prenatal test for
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| | challenge and there is a lot of
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| Downs Syndrome. This is why the recent
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| | excitement in the medical research world
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| announcement of a development of such a
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| | about this potential breakthrough.
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| test reported in the medical journal The
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| | In fact, this research work by Ravgen
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| Lancet (Feb. 3, 2007) has caused such a
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| | Inc. in the United States is not the only
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| stir.
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| | current development in this area.
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| Maternal Blood Test
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| | Researchers in Hong Kong are also
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| Dr Ravinder Dhallan of Ravgen Inc. has
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| | reportedly on the verge of a similar
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| reported results of a study to diagnose
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| | breakthrough but employing a different
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| Downs syndrome for the unborn baby using
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| | method which utilises placental DNA in
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| Maternal blood and therefore practically
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| | maternal circulation.
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| eliminating the potential risk of
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| | These methods might usher in a new era of
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| miscarriage associated with the available
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| | safe and effective prenatal diagnostic
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| invasive tests. What Dr Dhallan and
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| | tests. If the hurdles are overcome, the
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| colleagues did in this study was to
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| | rough estimate is that the tests may be
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| collect blood from 60 mothers whose
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| | available for use in about three years.
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