©Pregnancy bliss 2008
Diagnosing Down’s syndrome
A definite prenatal diagnosis of Down’s Syndrome for the baby in the womb requires an invasive test. The standard tests in use include amniocentesis and Chorionic Villi Sampling (CVS).
Whilst both tests are, in the right hands, relatively easy to perform, they do carry
a small but definite risk of provoking a miscarriage. This has always been the very
difficult question that prospective parents face when making a decision whether
to have the test or not. This has been the case. Until now.
Screening Tests for Down’s Syndrome
Screening test differ from diagnostic tests in
the sense that, the former only give you the degree of probability of the baby being
affected. The results are not definitive.
The screening tests which are non-invasive have varying degrees of detection rates
from a lowly 30% for maternal age alone to a clearly impressive 85% for a combination
of maternal age, fetal nuchal translucency (scan) and maternal serum biochemistry
at 11-14 weeks. This is called the Integrated Test.
Research has also shown that fetuses affected by Down’s syndrome have a missing nasal
bone on an ultrasound scan performed at 11-14 weeks. This test alone has been shown
to identify around 70% of Down’s syndrome babies. Crucially however, it did not appear
to give any advantage over the longer established nuchal translucency scan performed
at the same gestation and therefore the uptake has been poor.
One of the most frustrating
weaknesses of all screening tests is the fact that they also falsely detect unaffected
babies, occasionally leading to unnecessary invasive diagnostic tests and always
causing significant stress to prospective parents. The false positive rate of screening
tests is around 5%.
Moreover, even the most sensitive screening test will still miss
some babies that are affected and for all parents this means a little seed of doubt
until the baby arrives.
Diagnostic Tests for Down’s Syndrome
As mentioned earlier,
the definitive diagnostic tests mentioned (amniocentesis and CVS) suffer from one
Achilles heel: They are invasive and therefore carry the potential risk of provoking
miscarriage. This can happen regardless of whether the baby is affected by Down’s
syndrome or not. Because of this, there has been a lot of research activity to find
a risk-free reliable diagnostic prenatal test for Down’s Syndrome. This is why the
announcement of a development of such a test reported in the medical journal The
Lancet (Feb. 3, 2007) caused such a stir.
Amniocentesis carries an inherent risk of provoking a miscarriage