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Thursday, 26 May 2011

Down's Syndrome and other chromosome abnormality testing

With more women having children after the age of 35 years, testing for Down's Syndrome ( Mongolism ) and other chromosome abnormalities has become even more important. The test with the highest accuracy of course would be the amniocentesis, however this is an invasive procedure and it would not be practicle to perform this on every pregnant woman above 35 years. Testing has evolved from the triple test (measuring 3 markers in the blood at 15 weeks onwards ) to tests which are done in the first trimestor ie 11 -14 weeks.

In my practise, I will start with looking for the nasal bone between 11and 14 weeks. An absent nasal bone is strongly suggestive of a chromosome problem.  I will do two tests. The first is the measurement of the nuchal translucency ( this is the fluid at the back of the foetal neck which is present in all foetus's but is greatly increased in one with a chromosome disorder ). This should be <2.5mm. There are false positives and I increase the accuracy by combining this test with measurement of 2 markers in the blood, one a hormone beta-HCG and a protein PAPP-A. The combination of these tests is called the Oscar test. This gives a predictive accuracy of 95% for a chromosome abnormality.

If the tests are positive then an amniocentesis or CVS is offered. The alternative is to retest with a triple test and nasal bone measurement at 16 weeks, and if positive an amniocentesis should be done as the likelihood of a chromosome abnormality is high.

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