Down syndrome: prevalence at birth, mortality and survival. A 17-year study.

Analysis of comprehensive records over a period of 15 yr permits calculation of the total prevalence of Down Syndrome in an industrial city. The prevalence at birth is shown to have fallen from 1.70 per 1000 births in the period 1961--65 to 0.84 per 1000 births in 1971--75. The fall in prevalence can be shown to be largely due to a change in distribution of maternal age. An earlier increase in the number of Down Syndrome children is shown to reflect a greatly improved survival in the first 5 yr of life to a continuing level of 81%. The number of Down Syndrome children is unlikely to increase much even with increase of birth rate unless older mothers have further pregnancies. For these reasons the policies of amniocentesis of older women and termination of affected pregnancies will have progressively less effect on the number of affected children if this demographic trend continues. A further effect is that special schools will have a proportionately greater burden as the younger children being admitted have less prospect of development and may be more demanding of staff than Down Syndrome children.