Prodromal behavioral markers and developmental trajectories of autism spectrum disorder in infancy: a narrative review.

Autism Spectrum Disorder (ASD) is a neurodevelopmental condition characterized by social communication deficits and restricted, repetitive behaviors. Despite the rising global prevalence, a significant gap remains between the biological onset of the disorder and the average age of clinical diagnosis (3-5 years). This "diagnostic lag" hinders access to early intervention during the critical window of neuroplasticity. Here, we synthesize recent evidence regarding behavioral markers in the prodromal phase (0-12 months) and the symptom consolidation phase (12-24 months), with a specific focus on the heterogeneity of developmental trajectories. Unlike static disease models, ASD manifests through dynamic "developmental cascades." During the prodromal phase, non-specific signs-such as motor delays, attentional disengagement, and sensory regulatory issues-often precede overt social deficits. By the second year, these early vulnerabilities cascade into core symptoms, including the loss of joint attention, diminished response to name, and the emergence of restricted and repetitive behaviors. We further distinguish between "early-onset" and "regressive" patterns. Ultimately, by integrating empirical evidence with emerging digital phenotyping, we advocate shifting the clinical paradigm from "waiting for diagnosis" to "monitoring developmental trajectories," thereby optimizing early identification strategies to improve long-term outcomes for affected children and their families.