Lee and his colleagues analyzed data from 480,728 individuals in the Stockholm Youth Cohort, an ongoing study of children born between 1984 and 2007 in Stockholm, Sweden. Of the roughly 2 percent of children diagnosed with autism, 2,368 have autism with intellectual disability and 7,657 have autism only.
The prevalence of autism is highest — 52.4 per 1,000 — in babies born at 27 weeks of gestation. The rate goes down every week until 40 weeks, when it stands at 19.8 in 1,000. It picks up again in children born between 41 and 43 weeks, when it peaks at 23 in 1,000.
The preterm pattern is similar for children who have autism alone and those who have autism with intellectual disability. But postterm, only the rate for autism with intellectual disability rises significantly, and the rise is small: from 4.6 to 6.2 per 1000 children. The findings were published 12 September in Paediatric and Perinatal Epidemiology.