To identify possible early signs of autism spectrum disorder (ASD) within the Danish National Birth Cohort, we studied prospectively collected interviews from 76,441 mothers about their children's development and behaviour at 6 and 18 months. In Danish national registries, 720 children with ASD and 231 children with intellectual disability (ID) were identified. At 6 months, associations between early signs and ASD or ID were found only in few areas. At 18 months social, language, and motor skills were delayed, and suspicion of vision and hearing problems were increased for both groups. Signs distinguishing ASD from ID were unclear, and the positive predictive values regarding ASD were below 10 % for individual predictors and aggregated risk scores.