Outdoor play and catch-up growth in small-for-gestational-age children: a nationwide prospective birth cohort study from the Japan environment and children’s study

Abstract Background Children born small for gestational age (SGA) are at increased risk of impaired growth and adverse long-term health outcomes. While several prenatal and perinatal determinants of catch-up growth have been identified, evidence regarding modifiable postnatal behavioral factors remains limited. Outdoor play, which reflects both physical activity and environmental exposure, may influence linear growth during early childhood. This study aimed to examine the association between outdoor play and catch-up growth by age 3 years among children born SGA. Methods This nationwide prospective birth cohort study used data from the Japan Environment and Children’s Study (JECS). Among 104,043 registered fetuses, 1,796 children born SGA with available data on outdoor play and height at age 3 years were included. Caregivers reported children’s outdoor playtime during summer and winter between ages 1–2 and 2–3 years. Outdoor play was categorized as no outdoor play in either season, outdoor play in one season, or outdoor play in both seasons. The primary outcome was catch-up growth, defined as height ≥ − 2 standard deviations at age 3 years. Multivariable logistic regression models were used to estimate adjusted odds ratios (AORs) and 95% confidence intervals (CIs), adjusting for maternal, perinatal, and environmental factors. Results Among 1,796 children born SGA, 296 (16.5%) did not achieve catch-up growth by age 3 years. No outdoor play in either season between ages 1–2 years was associated with higher odds of failure to achieve catch-up growth compared with outdoor play in both seasons (AOR 2.51; 95% CI, 1.22–5.18). A similar association was observed between ages 2–3 years (AOR 4.36; 95% CI, 1.42–13.38). Outdoor play in only one season was not significantly associated with catch-up growth. Conclusions Outdoor play during early childhood was associated with catch-up growth by age 3 years among children born SGA. While these findings suggest that outdoor play may represent a potentially modifiable behavioral factor related to early growth, the observational nature of the study precludes causal inference. Further studies are needed to clarify underlying mechanisms and to determine whether promoting outdoor play can support healthy linear growth in this population.