Parental concerns correspond to earliest age of autism diagnosis in increased likelihood infant cohort

Autism spectrum disorder (ASD) encompasses a set of behavioural features with a diverse range of presentations, challenges, and trajectories. Previous research has demonstrated how behavioural and developmental markers can differentiate autistic from non-autistic children as early as the first year of life, however there is a dearth of literature demonstrating heterogeneity amongst the clinical presentations of autistic infants. An understanding of the breadth of experiences is necessary to refine and expand early identification methods to identify those currently being overlooked. The current study examined the heterogeneity of early behaviour in a longitudinal cohort of infant siblings of autistic children (<i>n</i> = 72) who later received an ASD diagnosis. Parent reports of early behaviour, generated at six time points between six and 24 months of age, describing the presence or absence of concerns across 10 developmental domains, were assessed to compare subgroups of infants based on the earliest age they were diagnosed as autistic (18, 24, or 36 months). The average number of concerns across domains, in addition to the proportion of each subgroup reporting a concern in each of the 10 domains, was compared at each time point. The infants diagnosed at 18 months had a higher average number of concerns across all time points compared to those diagnosed at 24 or 36 months, who demonstrated similar profiles. Play, language, and language regression concerns resulted in the largest effect sizes between groups. These findings indicate that (i) there is heterogeneity in early autism presentations, (ii) within the context of early identification, lack of diagnosis at one age does not eliminate the possibility of future diagnosis, and (iii) that parent reports of early concerns can provide valuable information that can alert clinicians to the features of autism, further attention to which may help reduce disparities in age of diagnosis.