Sex‐Specific Associations of Small, Vulnerable Newborns With Mental Capital in Early School‐Aged Children

BACKGROUND: Although mental capital is increasingly recognised as a foundation for children's later well-being, its perinatal determinants remain unclear because traditional classifications such as preterm birth and small for gestational age (SGA) capture only partial and overlapping aspects of vulnerability. OBJECTIVES: Using the integrated small vulnerable newborn (SVN) framework with mutually exclusive categories, we aimed to assess associations between SVN categories and mental capital at ages 6-7 years by sex. METHODS: We analysed data from six waves of the Adachi Child Health Impact of Living Difficulty (A-CHILD) study, a population-based repeated cross-sectional survey of first-grade students in Adachi City, Tokyo, conducted between 2015 and 2024 (n = 20,928). Caregivers provided gestational week and birthweight from the Maternal and Child Health Handbook, and children were classified into SVN categories (term non-SGA, term SGA, preterm non-SGA, preterm SGA). Mental capital, including behavioural problems, prosocial behaviour, and resilience, was assessed using the parent-reported Strengths and Difficulties Questionnaire and Children's Resilience and Coping Scale. Multivariable regression models were fitted, stratified by biological sex. RESULTS: Among the 20,928 children, 3402 (16.3%) were term SGA, 1093 (5.2%) were preterm non-SGA, and 227 (1.1%) were preterm SGA. Compared with term non-SGA, term SGA was associated with higher total difficulties scores among girls (standardised β 0.11, 95% confidence interval [CI] 0.05, 0.17) but not among boys (β 0.03, 95% CI -0.03, 0.09). Term SGA girls also showed lower resilience (β -0.07, 95% CI -0.12, -0.01). In contrast, preterm non-SGA boys had higher total difficulties scores (β 0.13, 95% CI 0.04, 0.21). There was no evidence that any SVN category was associated with prosocial behaviour in either sex. CONCLUSIONS: Term SGA tended to be associated with behavioural problems and lower resilience among girls, whereas preterm non-SGA tended to be associated with behavioural problems among boys.