Developmental Trajectories in Children With and Without Autism Spectrum Disorders: The First 3 Years

Retrospective studies indicate 2 major classes of autism spectrum disorder ( ASD ) onset: early and later, after a period of relatively healthy development. This prospective, longitudinal study examined social, language, and motor trajectories in 235 children with and without a sibling with autism, ages 6–36 months. Children were grouped as: ASD identified by 14 months, ASD identified after 14 months, and no ASD . Despite groups' initial similar developmental level at 6 months, ASD groups exhibited atypical trajectories thereafter. Impairment from 14 to 24 months was greater in the Early‐ ASD than the Later‐ ASD group, but comparable at 36 months. Developmental plateau and regression occurred in some children with ASD , regardless of timing of ASD diagnosis. Findings indicate a preclinical phase of varying duration for ASD .