Time trends of spina bifida in Sweden 1947-81.

The spina bifida rate in Sweden from 1947 to 1981 has been studied using various sources of information, including two central computerised registers. During the period the rate approximately halved, but the decline was not smooth and occurred in three "waves." When the geographical location of high risk areas within each wave was studied, they were found to differ. The findings are discussed in the light of an environmental aetiology hypothesis of this malformation.