No abstract

The epidemiology of neural tube defects was reviewed, using data from two birth defects surveillance systems: the nationwide Birth Defects Monitoring Program and the Metropolitan Atlanta Congenital Defects Program, for 1970-1978 and 1968-1979, respectively. After excluding cases with recognized causes, neural tube defects were divided into two major groups: "singles" and "multiples," depending on the presence of associated major defects. Only singles, which accounted for the majority of cases, were shown to have the well-known epidemiologic characteristics of neural tube defects: marked predominance of females and whites, geographic variation with an east-to-west gradient, and decreasing rates over time. On the other hand, multiples had no excess of females and occurred less predominantly in whites; moreover, their rates showed no geographic variation and little or no downward trends over time. The presence of associated defects indicates that neural tube defects are epidemiologically and probably etiologically heterogeneous. It is suggested that analytic studies of neural tube defects may be more rewarding if they try to identify different risk factors associated with various subgroups. This approach to the study of birth defects may provide better clues to their etiology and pathogenesis.

Our results indicate that categorizing cases of trisomy 21 by parent and timing of the meiotic error allows more precision in identifying risk factors and may shed light on mechanisms of meiotic nondisjunction.

We investigated the methods of recruitment, levels and determinants of response, and the calculation method of response proportions in population-based case-control studies in Germany. We identified 26 eligible studies. The pooled analysis showed that the response proportion was 68% among controls and 80% among cases. Response proportions among controls without telephones were about 50% lower than for controls with telephones. We propose a new measure that indicates the efficacy of the recruitment strategy.