We became aware that the objective of the screening programme (usually to detect as many cases of Down's syndrome as possible) differs from that of individuals being offered screening (to make a personal choice).
The chance of Down's syndrome in a woman in her early twenties is roughly one in 2,000, but screening now inevitably projects worries about the problem to younger as well as older women. The blood test is less sensitive (39 per cent pick-up at age 25) in the younger women who have the most babies, and the problems of false positive results are relatively greater. In effect, the existence of the test is responsible for raising anxiety but may be less than helpful. It may also contribute to more frequent accidental miscarriages of unaffected foetuses.
For older mothers the risk predicted by the blood test is often not sufficiently different from her age risk to help her make a decision about an amniocentesis.
We now routinely modify our initial discussion to take the mother's age into account. Respect for individual values can only be met by careful preliminary explanation, however difficult.
This will often require greater resources and may lead to a lower uptake of tests. If resources really are saved from community budgets by screening (by avoiding the costs of care for people with Down's syndrome), perhaps some of the savings might be directed back to improving screening programmes themselves.
ANDREW J. DAWSON, senior lecturer in obstetrics, University of Wales; TIM REYNOLDS, consultant in chemical pathology, Burton Group of Hospitals, Staffordshire; GILLIAN JONES, research midwife, University of Wales