Researchers identified all cases of Down's syndrome in eight district general hospitals in the Wessex region between 1994 and 1999. During these six years, 155,501 babies were delivered 335 cases of Down's syndrome were identified. Across the region, seven different screening policies were used, in three principal groups:
- serum screening
- screening by maternal age plus serum or nuchal screening
- screening by maternal age (35 years or more) combined with routine anomaly scans
Fifteen per cent of pregnant women were aged over 35 years at delivery - more than double the 5-7% presumed in statistical models of screening - and 58% of infants with Down's syndrome were born to women in this age range.
The team found no evidence that serum and nuchal screening improves antenatal detection rates or reduces rates of invasive procedure, such as amniocentesis. The districts that used serum screening detected 57% of cases, those using maternal age plus serum or nuchal screening detected 52%, and those using a maternal age of 35 or more and anomaly scans detected 54%.
Their findings also suggest that the recently announced government initiative to introduce universal serum screening from 2004 - to increase detection rates and reduce the need for invasive procedures such as amniocentesis - will not achieve its stated objectives.
"To avoid continuing the confusion that Down's screening currently causes in pregnant women, we believe that new screening methods should be offered only as part of a controlled study until their benefit is proved," they conclude.