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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.

Environmental factors remain undefined as possible causes of spina bifida and anencephaly. The authors noted a fivefold increased incidence for 1979-1980 in the Antioch-Pittsburg , California area, prompting a case-control study. They studied nine case and 27 control mothers as well as eight case and 17 control fathers to evaluate factors including residence, occupation, drug use, illnesses, and pesticide or chemical exposure. Except for fathers' smoking (p less than 0.05), no exposure was associated with neural tube defects.

Data from birth certificates (BC), death certificates (DC) and medical rehabilitation files (MR) were analyzed to estimate the livebirth prevalence of spina bifida in upstate New York in 1969-1974 and the completeness of the data sources. Birth certificates listed about 68% of cases, death certificates about 27% and medical rehabilitation files about 25%. The three sources together, it is estimated, included only about 80% of cases in the population. For each source, comparisons of estimates of completeness derived using each of the other two as reference sources were found to be useful for evaluating the likelihood of source dependence. The estimated livebirth prevalence rate, adjusting for incomplete reporting and the observed negative dependence of MR and DC sources, was 0.85 per 1000 livebirths by both Bernoulli census and log-linear methods. Taking into account in addition evidence for a BC-DC positive dependence, the resulting prevalence rate estimates were slightly higher, 0.88 per 1000 by log-linear methods and 0.90 per 1000 by the Bernoulli census approach. In view of the likely BC-DC positive dependence, it is suggested that Bernoulli census estimates derived using only these two sources without some ancillary third data source are likely to be biased to a false low figure. Nevertheless, estimates from BC and DC alone may still be useful in establishing that the prevalence rate is above some minimum figure, for example a "breakeven" prevalence rate, in cost-benefit analyses of a possible prevention program.

The prevalence and distribution of 991 cases of neural tube defects who were born in Utah in 1940-1979 are analyzed. The average prevalence at birth is 1.00 per 1000 births (live and stillbirths), which is similar to other states in the western United States. A decline in prevalence is observed during the past 10 years, although there is not a long-term secular trend for the whole time period. Examination of hospital records shows that 17.9% of neural tube defects were not reported on birth or fetal death certificates. The male/female sex ratio is 0.67. No seasonality is found for spina bifida cases; however, a bimodal distribution, with a substantial deficit in May, is observed for anencephaly. A significant negative correlation is found between neural tube defect prevalence and the percentage of individuals in each Utah county who are members of the Mormon Church. No association is found between levels of fallout exposure in Utah and regional variation in neural tube defects, and time-space clustering analysis demonstrates no significant clusters of neural tube defects in Utah.