BackgroundCommon mental disorders often emerge during childhood and adolescence, and their prevalence is disproportionately elevated among those affected by obesity. Early life growth patterns may provide a useful target for primordial prevention; however, research is lacking. Therefore, this study aimed to identify distinct body mass index (BMI) trajectories during the first year of life and to assess their associations with psychosocial outcomes in preadolescence (9–13 years).MethodsData were obtained from n = 1778 Greek children (9–13 years). Infant anthropometric data were obtained from paediatric health records and BMI trajectories during the first year of life were estimated using group-based trajectory modelling. Preadolescent emotional functioning, self-esteem, body image dissatisfaction and dieting behaviours were self-reported via validated questionnaires. Associations were estimated using binary and ordinal logistic regression, adjusted for key confounders.ResultsFour BMI trajectories were identified: low (26.7%), average (41.8%), high (25.2%), and very high (6.4%). Children belonging to the very high trajectory had greater odds of body image dissatisfaction (OR: 1.62, 95%CI: 1.11, 2.38), dieting behaviour (OR: 1.49, 95%CI: 1.01, 2.20) and restrained eating (OR: 1.69 95%CI: 1.14, 2.52) than children belonging to the average trajectory. Body image dissatisfaction was also greater in children belonging to the high trajectory (OR: 1.40, 95%CI: 1.11, 1.76). However, infant BMI trajectories did not significantly predict childhood emotional functioning or self-esteem status.ConclusionInfants with BMI growth in the high reference ranges had poorer psychosocial outcomes in preadolescence. Whilst further research is needed to replicate these findings, monitoring early infant growth trajectories may allow for early stratification of infants at risk of poor psychosocial outcomes.