Autism spectrum disorder: updated prevalence and comparison of two birth cohorts in a nationally representative Australian sample

OBJECTIVES: This study aimed to (1) provide an update on the prevalence of parent-reported autism spectrum disorder (ASD) diagnosis and new information about teacher-reported ASD in two nationally representative Australian cohorts at ages 10–11 years, (2) examine differences in cohort demographic and clinical profiles and (3) compare the prevalence of teacher-reported ASD and any changes in categorisation over time across the cohorts. DESIGN: Secondary analyses were undertaken using data from the Longitudinal Study of Australian Children (LSAC). PARTICIPANTS: Children were recruited at kindergarten age (K cohort; birth year 1999/2000) and birth (B cohort; birth year 2003/2004), with follow-up of every 2 years for six waves. PRIMARY OUTCOME MEASURES: Parent-reported and teacher-reported ASD diagnosis was ascertained at three time points (waves 4–6). RESULTS: At age 10–11 years, the adjusted prevalence of parent-reported ASD diagnosis was 3.9% (95% CI 3.2 to 4.5) and 2.4% (95% CI 1.6 to 2.9) in the B and K cohorts, respectively. Teacher-reported prevalence of ASD was 1.7% (95% CI 1.2 to 2.1) in the B cohort and 0.9% (95% CI 0.56 to 1.14) in the K cohort. Parents reported fewer conduct and peer problems and teachers more pro-social behaviour in B relative to K cohort ASD children. Children reported only by parents in the later-born B cohort had milder behaviour problems than parent-agreed and teacher-agreed cases. Although individual switching to ASD from other categories from 8–9 to 10–11 years was low (K cohort n=5, B cohort n=6), teachers reported more children with ASD in the B than K cohort at 10–11 years and fewer children with emotional/ behavioural problems. CONCLUSIONS: The higher prevalence of parent-reported and teacher-reported ASD diagnosis in the later-born cohort may be partially explained by identifying children with milder behavioural problems as ASD and a change in the use of diagnostic categories in schools.