International variation in the prevalence at birth of anencephalus in relation to maternal factors.

Information from series of cases of anencephalus and corresponding random samples of all livebirths showed that the prevalence rate at birth per 1000 livebirths for anencephalus was 4.02 in Belfast (from 1957 to 1969), compared to 1.36 in mothers of Scots-Irish origin resident in 14 selected Canadian cities (from 1950 to 1969). For each of these two populations, anencephalus was associated with the number of previous livebirths, stillbirths, and child deaths. However, these maternal factors did not account for any appreciable portion of the difference in prevalence rates between the two populations, showing that a different set of factors must cause the international difference in rates.