Secular changes in the incidence of malformations of the central nervous system.

In a previous communication (Record and McKeown, 1949) it was noted that in Birmingham the incidence of anencephalus and spina bifida decreased during the years 1940-47, while the incidence of hydrocephalus remained fairly constant. It was tentatively suggested that this decrease might be correlated with changes in the parity distribution of births, since it was shown that the incidence of anencephalus and spina bifida is associated with birth order. We propose here to examine this suggestion more critically, making use of statistics for additional years and of more detailed information than was previously available about the parity distribution of the general population of births. Table I (see Fig. 1) gives the incidence of malformations of the central nervous system notified as stillbirths or infant deaths in Birmingham for the period 1936-49. Anencephalus associated with spina bifida is classified under anencephalus, and spina bifida with hydrocephalus under spina bifida. During the years 1940-49 the incidence of anencephalus and spina bifida fell sharply; the incidence of hydrocephalus showed no consistent variation. The significance of these changes