Area differences in prevalence of neural tube malformations in South Wales. A study of possible demographic determinants.

In England and Wales as a whole the frequency of neural tube malformations is highest in the north and north-west, and lowest in the east, south-east, and south (Rogers, 1969). Differences have also been reported in the regional frequency of anencephalus, spina bifida, and hydrocephalus in Scotland (Edwards, 1958), and Hewitt (1963) found that the mortality attributed to spina bifida was two to three times greater on the Atlantic than on the Pacific coasts in both Canada and the U.S.A. (in the latter among the white population only). While the significance of differences from country to country may be difficult to interpret, the presence of regional differences within a country and, more particularly, area differences within a region are strong indicators of environmental influence. Marked local variation in the prevalence of neural tube malformations in South Wales was first reported by Laurence, Carter, and David (1968) in a study (covering a six-year period 1956-62) confined to the agricultural Vale of Glamorgan and the mining valleys of Glamorgan and Monmouthshire, an area with a total population of approximately 850,000 and a birth population during the survey period of 102,786. Laurence et al. (1968) examined possible geographical and climatic factors but none was thought to be related to the local variation in malformation prevalence. More recently, Lowe, Roberts, and Lloyd (1971) and Richards (1971) confirmed the presence of significant area differences in a large study covering the whole of South Wales. Lowe et al. found that the