Autism spectrum disorder (ASD), a common neurodevelopmental disability, typically can be diagnosed within the first 3 years of life, usually by age 24 months. The median age of diagnosis in the United States is 4.5 years, which is substantially later than when symptoms emerge.¹ Despite increasing public awareness, only 43% of children with ASD receive a comprehensive developmental evaluation by age 3 years.¹ Timely detection facilitates access to appropriate developmental and behavioral intervention that, when begun soon after symptom onset, can substantially improve outcomes and reduce parents’ stress.

Autism spectrum disorder involves core impairments in social communication and the presence of stereotypic, restricted, and repetitive behaviors. One in 59 children (1.7%) in the United States had ASD in 2014, a 2.5-fold increase from the rate in 2000.¹ At least in part, this increase reflects better detection, including diagnosis of children with higher IQs, those with milder ASD symptoms, and members of racial/ethnic minority groups, who may have been previously overlooked or given other diagnoses. Prevalence rates also vary by US state and the ascertainment method used to estimate prevalence.^1,2 There is marked heterogeneity in ASD presentation, with some individuals exhibiting prominent repetitive motor behaviors while others may have intense interests. Some individuals with ASD are minimally verbal, while others speak fluently but with poor communication skills; intellectual functioning can be impaired or far greater than the mean. In addition, ASD has a strong genetic basis, with between 60% and 90% of the heterogeneity in symptom expression attributable to genetic variation, reflecting both common allelic variants and structural variation in coding and noncoding regions of DNA.³ Younger siblings of children diagnosed with ASD (siblings at high risk) have an approximately 12-fold increased risk for ASD. Brain imaging studies demonstrate subtle alterations in white matter development, functional connectivity, and cortical growth prior to the emergence of core ASD symptoms in siblings at high risk who are subsequently diagnosed with ASD.^4,5