Data from the Finnish Register of Congenital Malformations for the years 1965-73 were used in a search for associations between environmental influences and defects of the central nervous system (CNS). The material consisted of 710 cases of CNS defects and their matched-pair controls. Moreover, and 'internal' control group of 259 cases of polydactyly and their matched-pair controls were used. The first report gives information on variations in time and space, sex distribution, and parental age. A higher incidence of anencephaly was noted in the eastern part of the country, but no significant secular or seasonal variations were found. The sex ratio (M/F) was lower than expected in the groups of anencephaly and CNS defects as a whole. High parental age turned out to be a risk factor in the group of all CNS defects, mainly owing to the subgroup of hydrocephaly. The dangers of observational studies due to confounding factors are discussed.
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