The ante-natal detection of Down's syndrome: some demographic aspects.

The numbers of Down's syndrome live births in the area covered by the Sheffield Regional Hospital Board (Trent Regional Health Authority) were estimated using age specific incidence data from a recent detailed Swedish study. The estimated number and proportion of Down's syndrome live births, which could have been detected by prenatal screening of elderly mothers at high risk, fell substantially in recent years. In the maternal age group forty to forty-four the estimated number of Down's syndrome livebirths decreased from thirty-three (24.6 per cent) in 1963 to thirteen (15.7 per cent) in 1973. Over the same period there was a fall in estimated numbers in the thirty-five to thirty-nine age group from twenty-seven (20.2 per cent) to twelve (14.5 per cent). This trend has been due predominantly to a rapid decline in the fertility rates of women aged thirty-five and over. However, projections based on the best available evidence suggest that about 11.4 per cent to 13.8 per cent of all Down's livebirths up to 1991 could be detected by screening all pregnancies in the forty to forty-four age group. A further 11.7 per cent to 14.4 per cent of Down's livebirths could be detected by screening pregnancies in the thirty-five to thirty-nine age group.