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From 1 January 1997 to 31 December 2001 we performed a retrospective observational study on the incidence, accuracy of clinical diagnosis, and prevalence estimation of Down's syndrome in a well defined population of 1.7 million in Northern Ireland. 1
A total of 208 postnatal cases of Down's syndrome were diagnosed: 197 trisomy (94.7%), 3 translocation (1.45%), and 8 mosaic cases (3.85%) (expected ratios 94% trisomy, 5% translocation, 1% mosaic 2 ). In a population of 114 307 live births, a minimum prevalence of 167.9 per 100 000 (or 1 in 595 births) was calculated.
The detection rate of mosaic variants is higher than quoted rates of 1-3%. This may be accounted for by inclusion...