Risk factors for mental retardation were studied prospectively in 12 000 children born in northern Finland in 1966 and followed to the age of 14 years. The number of untraced children was less than 2 per 1000. Altogether 326 children had an IQ less than 86, and the incidence of severe retardation (IQ less than 50) was especially high. An incidence figure for children with mental retardation, a separate figure for healthy children, and also the death rate were calculated for each disease. Only in the cases of Down's syndrome and some hereditary diseases were all the exposed children mentally retarded; in other diseases some children did not seem to suffer any sequelae. A risk factor could be found for 50.6% of the total number of children with mental retardation, the percentage decreasing from the severest to the mildest form (86.7%, 45.4%, and 30.9%). Some 9.4% of the healthy children and 77.7% of those who died had had one or more of these conditions. Prenatal conditions were most often associated with severe mental retardation (64%), and perinatal conditions with mild retardation, (IQ 50 to 70; 27%) and mental subnormality (IQ 71 to 85; 18%). Cases with no known risk factor were more common among boys than girls.
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