Mild mental retardation in northern Finland.

The occurrence and aetiology of mild mental retardation (IQ 50-70) and borderline retardation (IQ 71-85) were studied on the basis of a one-year (1966) birth cohort of 12,058 live-born infants, which were followed prospectively up to the age of 14 years. The cumulative incidences for MMR and borderline retardation were 5.5 and 13.4 per 1,000 respectively. In the borderline group there was a male preponderance of 2.3:1 as compared to 1.2:1 in MMR children. A causal factor could be identified for 46.9% of MMR children and 31.4% of borderline ones. Prenatal factors had very little bearing, since only 6% of the MMR and 6.7% of the borderline cases had such, as compared to 28.7% and 20.4% for perinatal factors and 12.1% and 4.3% for postnatal factors. An analysis of the impact of specific obstetrical and perinatal factors revealed that very few constituted relevant relative risks for mental retardation.