Area differences in spontaneous abortion rates in South Wales and their relation to neural tube defect incidence.

Data are presented from the South Wales Congenital Malformation Survey (92,982 births 1964-6 inclusive) showing that within areas in South Wales there exists an inverse relation between previous spontaneous abortion rate and the prevalence at birth of neural tube defect (anencephaly or spina bifida cystica or both). This relation is independent of social class, parity, and maternal age, and is not likely to be explained by area differences in accuracy of reporting previous spontaneous abortions.On the basis of these findings a hypothesis is advanced which proposes that the incidence of neural tube defects is uniform throughout South Wales and that the present substantial and relatively stable differences in area prevalence are controlled by small area differences in mortality of malformed embryos. This would seem to suggest that factors initiating the malformation are genetic and that any related environmental factors exert their effect on already abnormal fetuses by influencing, in one way or another, their capacity to survive.