Note: Of the children invited to participate in the ABC study based on selection criteria, one child did not complete the assessment and one child had Rett syndrome. Both were therefore excluded. ASD=autism spectrum disorder. NDD=neurodevelopmental disorders without ASD. IQ=intelligence quotient. MoBa=the Norwegian Mother, Father and Child Cohort Study. Of the children without any clinical diagnosis (n=454), 444 children had data on walking and IQ. ASD included ABC Protocol ASD diagnosis as of 19 Oct 2014 (Autistic Disorder, Asperger Disorder and Pervasive Developmental Disorder Not Otherwise Specified), childhood disintegrative disorder and profound disability with autism.*Rett syndrome was excluded (n=2) because of insufficient information on whether ASD diagnostic criteria were met (i.e., for either Autistic Disorder or PDD-NOS).