["Autism Research, EarlyView. ", "\nABSTRACT\nSeveral studies have examined trajectories of autism symptoms in autistic children and their siblings. However, less is known about developmental patterns across different neurodevelopmental conditions, as well as the clinical characteristics and early predictors among children with distinct diagnoses but overlapping trajectories. This study investigated trajectories of autism symptoms [ADOS‐2 Calibrated Severity Score; Total CSS, social affect (SA) CSS, and restricted and repetitive behaviors (RRB) CSS] and their overlap across symptom domains in a Chinese cohort of 163 children aged 18–36 months, including autism, broader autism phenotype (BAP), developmental delay (DD), and typical development (TD). Latent class growth modeling identified three trajectories for Total CSS and SA CSS, and two for RRB CSS. Cross‐domain analyses revealed overlapping CSS trajectories between 51 autistic children (Overlap‐Autism) and 36 children with BAP (Overlap‐BAP). Clinical comparisons between these two groups revealed distinct temporal profiles in CSS and ADI‐R scores. Hierarchical logistic regression analyses indicated that female sex was a protective factor for Overlap‐Autism (OR = 0.20, p = 0.009) relative to the Overlap‐BAP group, whereas higher SA CSS at 18 months independently predicted Overlap‐Autism (OR = 1.61, p = 0.003). These findings highlight the challenges of early diagnostic differentiation among children with autism‐related traits and underscore the importance of comprehensive and longitudinal assessment.\n"]