Mental retardation in over half-a-million consecutive livebirths: an epidemiological study.

Prevalence and incidence of all levels of mental retardation were investigated in the population of British Columbia. Ascertainment was through the British Columbia Health Surveillance Registry. The minimum prevalence for all levels of retardation among the 15- to 29-year age group, where ascertainment was best, was 7.7 per 1,000 (8.8 per 1,000 males, 6.6 per 1,000 females). Minimum ascertainment (incidence) of retardation in the 1952 to 1966 birth cohort was 9.3 per 1,000 livebirths (9.9 per 1,000 males, 8.6 per 1,000 females). Data were also presented on lagtime (time from birth until a retarded person is identified in association with the level of functioning and the presence or absence of disabilities in addition to mental retardation.