As more and more women put off having a family until they are over 25 it is to be expected that the number of cases of Down syndrome will rise as a result of the well known maternal age effect. That this is already happening, in some places at least, is clearly shown in a recent study by Alan J Staples and his colleagues at the Adelaide Children’s Hospital (Staples et al, 1991).
In their study of the epidemiology of Down syndrome in South Australia between 1960 and 1989 the researchers found that the population incidence of the disorder was 1.186 births per thousand live births when taken over the whole period. However, they observed a distinct rise in the maternal age distribution during this period and they estimate that if this trend continues then the population incidence in South Australia will exceed 1.5 births per thousand live births in the five years from 1990 to 1994.
At present in South Australia amniocentesis or chorion villus sampling is available to women aged 35 and over and about half of them choose to be tested. It is intended in the future to screen all pregnant women using the results of an analysis of maternal serum taken in the second trimester together with their age to produce a risk figure for the birth of a child with Down syndrome. Those women with a high risk will be offered amniocentesis and it is believed that this technique will detect over 60 per cent of Down syndrome pregnancies.
Once this screening programme is in operation, assuming that a reasonable number of women shown to be carrying a child with Down syndrome elect to have their pregnancy terminated, the number of live births with the disorder should fall. Until then a rise in the number of children born with Down syndrome seems inevitable.
Reference: Staples et al, Amer. J. Hum. Genet. v.49, p1014, 1991.
John Timson