Screening for Down's syndrome

Ignorance abounds

Down's syndrome (trisomy 21) is the commonest cause of mental retardation and is also associated with major structural anomalies of the heart and gastrointestinal system and an increased risk of leukaemia. Early mortality is high, mainly due to congenital heart defects, but between infancy and 40 years of age mortality is similar to the norm. Most surviving adults go on to develop brain abnormalities that are typical of Alzheimer's disease.

At birth the incidence of the syndrome is 1.3 per 1000, with the risk rising with increasing maternal age. Until recently screening was confined to women over an arbitrary age limit of 35 to 37 and entailed amniocentesis or chorionic villus sampling. These procedures carry a small but important rate of miscarriage. On this basis, one in 20 pregnant women would be offered screening, and the risk of giving birth to a baby with Down's syndrome would be comparable to the risk of miscarriage due to the procedure. This screening programme was based on …