Short-term changes in the incidence of malformations.

On at least two occasions, the examination of secular changes in the incidence of malformations has led to the identification of important aetiological agents. The discovery that rubella could cause congenital heart disease and cataract (Gregg, 1941) was based on studies of the incidence of these malformations. Similarly, the association between the taking of thalidomide in pregnancy and ectromelia was first demonstrated by inquiries carried out because defects of this type had become commoner (Lenz, 1961; McBride, 1961). The increases in incidence caused by rubella and thalidomide were unnoticed until they became conspicuous enough to impress clinicians. It is clearly desirable that epidemics of malformations should be recognized before reaching such proportions, and there is therefore a need for the examination of secular trends in large populations of births. In the present communication, data for Birmingham children born in 195741 are analysed in this way.