The Chief Scientist reports.... Pattern of decline in prevalence of anencephaly and spina bifida in a high risk area.

The results of three studies spanning the period 1964 to 1989 were aggregated in an attempt to identify secular trends which might contribute to aetiological understanding of anencephaly and spina bifida (ASB). All data related to the prevalence of ASB in a geographically defined population in Glasgow, a known high risk area of neural tube defects. Multiple sources of ascertainment were employed to identify affected cases, whether live births, still births or induced abortions following prenatal diagnosis. The birth prevalence of ASB dropped by 82%, from 5.63 per 1,000 births in 1964-1968 to 1.04 per 1,000 births in 1979-1989, while the pregnancy prevalence (adjusted by including induced abortions) declined by 46%, from 5.63 per 1,000 births in 1964-1968 to 3.02 per 1,000 births in 1979-1989. Thus, prenatal screening contributed just under half of the observed decline in ASB birth prevalence. The pregnancy prevalence appeared to decline throughout the 1970's and early 1980's, and to increase again, temporarily, in the mid-1980s. These data could be interpreted as being broadly consistent with socio-economic hypotheses of ASB aetiology.