Twenty-year birth prevalence of Down syndrome in Cape Town, South Africa.

The 20-year birth prevalence of Down syndrome in Cape Town, South Africa, was determined. All cases delivered to mothers in Cape Town, plus terminations following prenatal diagnosis, between 1 January 1974 and 31 December 1993 were ascertained. There were 784 Down syndrome pregnancies, of which 95% were trisomies. The 32 terminations comprised 18.3% of the white, 5.8% of the coloured (mixed race) and 1.4% of the black cases. The overall prevalence rate was 1.49 per 1000 (white 1.88, coloured 1.54 and black 1.29 per 1000). Analysis for linear trends showed a significant decline in rates for the total population and for whites, a downward trend for coloureds, but no decline for blacks. Over the last 5-year period the prevalence rates in all three population groups were 1.3 per 1000. An increasing risk with advancing maternal age was confirmed, but no maternal age-specific differences in rates by race were demonstrated.