The proportions of Down's syndrome livebirths associated with a Robertsonian translocation inherited from a carrier parent were estimated from data in the New York State Chromosome Registry and in two previous publications. Indirect estimates were made in each 5-year maternal age interval; these were derived from mutation rates for these translocations and maternal age specific prevalence rates in livebirths. The proportions diminished steadily with increasing maternal age. The ranges for the seven maternal age groups from under 20 to 45 to 49 were: 1.1 to 2.8%, 1.0 to 2.7%, 0.7 to 1.8%, 0.5 to 1.3%, 0.2 to 0.4%, 0.05 to 0.1%, and 0.02 to 0.04%. Direct estimates from the observed data could only be attempted for two age groups, women under 30 and those 30 or more. For those under 30 the range in proportions was 0.9 to 1.9% and for those 30 and over, 0.2 to 0.4%. In general the lowest proportions at any age were derived from New York State data and the highest from Japanese data.
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