An epidemiologic study of neural tube defects: Delta County, Michigan, 1969-1988.

We determined the incidence of neural tube defects (anencephaly, spina bifida, hydrocephalus) in a rural county in the Upper Peninsula of Michigan because several local physicians perceived that the incidence in this area might be high. To assure that all neural tube defects that occurred were noted, data for the study were collected from hospital birth-log books, patient records of all neural tube defect births, all stillbirth records, and from records of babies transferred to other regional hospitals. A reference group consisting of 10% of all nonneural tube defect births during the 20 years of the study was selected at random and used to make comparisons to the case group. During the twenty-year period 1969-1988, the incidence of neural tube defects (NTDs) was found to be 2.09 per 1000 total births. The incidence of anencephaly was 1.05/1000; of spina bifida, 0.56/1000; and for hydrocephalus, 0.48/1000. Seasonal variations by month of conception were noted for anencephaly and spina bifida, confirming previous findings in the literature. Study results showed that between 1972 and 1976 there was a cluster of NTDs, but the NTD incidence was not statistically different from what might be expected over a twenty-year period. The incidence of NTDs in Delta County was found to be normal for this part of North America. The results of this study indicate that physicians should be cautious when attempting to interpret rare events, which often occur in clusters, leaving the impression that their incidence is excessive.