Aetiological factors and prevalence of severe mental retardation in children in a Swedish municipality: the possible role of consanguinity.

The prevalence of severe mental retardation (SMR) was studied in one of the 24 suburban municipalities in Stockholm. The study area had a high proportion of non-European nationals. The study population comprised 14138 children born between 1979 and 1992 who resided in this municipality on the census day, 31 December 1995. The total prevalence of SMR was 4.5 per 1000, being 3.7 per 1000 and 5.9 per 1000 in the European and in the non-European population, respectively. The majority of cases (66%) had a definite prenatal origin. Down syndrome was the cause in 20%. Six families (10%) had at least two children with SMR. It was concluded that the prevalence was higher than in previous Swedish studies. Many cases were attributed to genetic factors. Consanguineous marriages were assumed to be a factor of importance in the distribution of aetiologies. Demographic differences between areas in Sweden must be considered when planning habilitation services.