Preterm birth and maternal psychological distress are associated with an increased risk for social competence difficulties; however, little is known about how this risk changes over time and across gestational age. This study aimed to examine mean developmental trajectories of social competence difficulties from early childhood to mid-adolescence by gestational age groups: very preterm (VP, <32 weeks), moderate-to-late preterm (MLP, 32-36 weeks), early-term (37-38 weeks) and full-term (39-41

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... ks) and to assess the mediating effect of maternal psychological distress during infancy on the social competence difficulties trajectories. Methods: Data were analysed on 15,821 participants from the UK Millennium Cohort Study participants, a nationally-representative birth cohort. Social competence difficulties were assessed by parent report when the participants were aged 3, 5, 7, 11 and 14 years. Maternal psychological distress was self-rated when the children were 9 months of age. Data were modelled using latent growth curve analysis. Results: The developmental trajectories of social competence difficulties were u-shaped in all groups showing a decline from age 3 to 7, followed by a stable low period and then an increase during adolescence. VP children (n=173) showed pronounced difficulties throughout (b=0.94, SE=0.36 at age 14). MLP children (n=1,130) and early-term children (n=3,232) showed greater difficulties compared with their full-term peers around age 7, which resolved by age 14 (b=0.20, SE=0.13; 0.03, 0.07, respectively). Maternal psychological distress during infancy mediated 20% of the above association for VP. Conclusions: The effect of gestational age on developmental trajectories of social competence difficulties can be dose-response. Monitoring and providing support on social development throughout childhood and into their adolescence and treating early maternal psychological distress may benefit preterm children, particularly those born VP.