Later-born siblings of children with autism spectrum disorder are considered at biological risk for autism spectrum disorder and the broader phenotype. Early screening may detect first signs of autism spectrum disorder and expedite diagnosis and early intervention. In this follow-up study, we re-examine a validated second-degree autism spectrum disorder screener for children at biological risk of autism spectrum disorder, the parent observation of early markers scale. Using available follow-up information up to age 13 years, 110 children (the original 108 infants plus 2 infants recruited after the completion of the original study) were divided into three groups: diagnosed group (n = 13), lost diagnosis group (n = 5), and undiagnosed group (n = 92). The parent observation of early markers scale continued to show acceptable predictive validity. The parent observation of early markers scale total scores and mean number of elevated items were significantly higher in the diagnosed group than the undiagnosed group. The lost-diagnosis group did not differ from the undiagnosed group on parent observation of early markers scale total scores and elevated items at any age, but the lost-diagnosis group had significantly lower total scores, and number of elevated items than the diagnosed group starting at 18 months. Both autism spectrum disorder core and subsidiary behaviors differentiated the diagnosed and undiagnosed groups from 9 to 36 months of age. We hereby provide further evidence that the parent observation of early markers scale may serve as a low-cost early screener for autism spectrum disorder in at-risk children and pinpoint specific developmental ,and behavioral problems that may be amenable to very early intervention.