Incidence study of Down's syndrome in Copenhagen, 1960–1971: with chromosome investigation

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Abstract
Incidence figures for Down''s syndrome were obtained throughout a period where a considerable change in the age distribution of child-bearing mothers has taken place. An expected fall in incidence was evaluated. In parts of the Copenhagen Metropolitan area [Denmark] 235 liveborn patients with Down''s syndrome were ascertained in the period 1960-1971 in a population of 1.2 million with a total of 204,771 births. All patients available were examined cytogenetically (75%). In 160 (90.4%) a regular trisomy 21 was observed. In 6.2% of the cases translocations and in 2.3% of the cases mosaics were found. Two double trisomies and a double trisomy mosaic were observed. Throughout the period 1960-71 the percentage of women over 30 yr delivering children decreased from 23.4% in the beginning of the period to 16.2% at the end of the period. In the 1st part of the period, 52.6% of the cases were born to mothers over 30; at the end of the period, 40% of Down''s syndrome mothers were of that age. However, the incidence was unchanged throughout the whole period, about 1.15/1000 births. For some age groups a steady rise in incidence of trisomy 21 cases was found throughout the whole period. A better ascertainment of the patients at the end of the period is required. Environmental factors may also play a role.