Background
Early motor and language milestones are key indicators of neurodevelopment, yet their associations with later autism traits and attention-deficit/hyperactivity disorder (ADHD) symptoms remain poorly understood. Neurobiological mechanisms linking early developmental delays to these conditions are not well established. Here, we investigate associations between early developmental milestones, resting-state functional connectivity (rsFC), and neurodevelopmental traits in a large adolescent sample.

Methods
Data were drawn from the Adolescent Brain Cognitive Development study (ABCD) and included 6,663 participants (age range = 8.92–11.08 years; mean age = 9.92 years; 48.42% female). Developmental measures included roll over, sit without assistance, walk without assistance, speech development, gross motor development, and coordination. Autism traits were assessed using the Social Responsiveness Scale, and ADHD symptoms were assessed using the Attention Problems subscale of the Child Behavior Checklist. rsFC was examined within and between cortical and subcortical regions. Linear mixed-effects (LME) models and Bayesian mediation analyses assessed associations between developmental measures, rsFC, and neurodevelopmental traits.

Results
LME models revealed shared and condition-specific associations. Poor coordination was associated with higher autism traits and ADHD symptoms, whereas delayed speech development was associated with autism traits, and delayed walking without assistance with ADHD symptoms. We found negligible evidence that rsFC measures mediated associations between early developmental milestones and neurodevelopmental traits. Indirect effects were modest, accounting for less than 2% of associations with autism traits, although somewhat larger effects were observed for ADHD symptoms.

Conclusions
These findings suggest overlapping developmental pathways, with intrinsic connectivity reflecting downstream neural processes rather than primary mediating mechanisms.