Although early autism spectrum disorder (ASD) detection strategies tend to focus on differences at a point in time, behavioral symptom trajectories may also be informative.
Developmental trajectories of early signs of ASD were examined in younger siblings of children diagnosed with ASD (n = 499) and infants with no family history of ASD (n = 177). Participants were assessed using the Autism Observation Scale for Infants (AOSI) from 6 to 18 months. Diagnostic outcomes were determined at age 3 years blind to previous assessments.
Semiparametric group-based modeling using AOSI scores identified three distinct trajectories: Group 1 (‘Low’, n = 435, 64.3%) was characterized by a low level and stable evolution of ASD signs, group 2 (‘Intermediate’, n = 180, 26.6%) had intermediate and stable levels, and group 3 (‘Inclining’, n = 61, 9.3%) had higher and progressively elevated levels of ASD signs. Among younger siblings, ASD rates at age 3 varied by trajectory of early signs and were highest in the Inclining group, membership in which was highly specific (94.5%) but poorly sensitive (28.5%) to ASD. Children with ASD assigned to the inclining trajectory had more severe symptoms at age 3, but developmental and adaptive functioning did not differ by trajectory membership.
These prospective data emphasize variable early-onset patterns and the importance of a multipronged approach to early surveillance and screening for ASD.

© 2021 Association for Child and Adolescent Mental Health.