The effect of differences in the distribution of maternal age in England and Wales on the performance of prenatal screening for Down's syndrome.

Abstract

We aimed to determine how differences in the age at which women had their pregnancies influenced the expected detection and false-positive rates of serum screening for Down's syndrome (i) between 1970 and 1993 in England and Wales, and (ii) between regions and districts of England and Wales in 1991. We applied published estimates of Down's syndrome screening to regional and district data on the age distribution of maternities and changes in the age distribution over time in England and Wales obtained from the Office of Population Censuses and Surveys. From 1970 to 1993 women, on average, became pregnant at an older age; the percentage of maternities among women aged 35 or more increased from 7 to 9.2 per cent. This was not a great enough change to have had a material effect on the performance of Down's syndrome screening. In 1991, the percentage of maternities among women aged 35 or more varied from 5 to 20 per cent among Health Districts, a difference that would influence the performance of screening; for example, using the triple test and a risk cut-off of 1 in 250, the detection rates would have varied from about 55 to 70 per cent and the false-positive rates from 4.4 to 8.8 per cent across different districts. The tendency for women to have their pregnancies at an older age would have had a negligible impact on the performance of serum screening for Down's syndrome, but differences in the age when women had their pregnancies in different parts of the country would lead to twice as many women being referred for amniocentesis in some districts than in others when offered the same method of serum screening and at the same risk cut-off level. The results will have important implications for the local purchasers of Down's syndrome screening services.