Developmental Milestones for Children With Down Syndrome: Revised Estimates Using Moving Average Summaries.

OBJECTIVE: Children with Down syndrome follow distinct developmental trajectories that require specialized monitoring and counseling. We aimed to provide updated estimates of developmental milestone attainment using a nonparametric approach and to compare these results with previously reported generalized linear mixed-effects model (GLMM) estimates. METHODS: We reanalyzed developmental data from 842 children with Down syndrome (ages 2 months to 24 years). For each milestone, achievement rates were calculated within overlapping 0.5-year time windows, and shape-constrained additive models were used to derive monotonic regression curves. RESULTS: Compared with GLMM-based estimates, the nonparametric approach predicted milestone attainment at least 1 year earlier for 17 of 25 milestones at the highest comparable percentile. Early gross motor milestones showed consistent achievement patterns across methods, whereas more complex adaptive, language, and academic skills demonstrated greater variability and were more sensitive to modeling assumptions. Females achieved 17 of 25 milestones at least 1 year earlier than males. CONCLUSIONS: These findings highlight substantial developmental heterogeneity and support the potential for continued skill acquisition throughout childhood and adolescence in individuals with Down syndrome. The updated estimates provide clinically relevant reference points to guide individualized monitoring, anticipatory guidance, and family counseling.