Age-related changes in the effect of birth weight on child development: findings from a Japanese Longitudinal Survey

The relationship between health status at birth and short-term health has been established; however, the relationship with long-term health remains unclear, particularly in Japan. This study investigates the effect of birth weight on child development using data from the Longitudinal Survey of Newborns in the twenty-firstst Century—a nationwide birth cohort study conducted by the Ministry of Health, Labour and Welfare. We employ twin fixed-effects estimation to examine the effects of birth weight on physical development, using measures including weight, height, percentage of overweight indexes, and hospitalization, with a focus on how the effects vary with age up to 12 years. The results show that for all ages, lower birth weight negatively affects body size but does not affect probability of hospitalization or overweight/underweight status, indicating a limited impact. However, lower birth weight shows larger adverse effects on body size and the probability of hospitalization when the sample is limited to birth weight < 2300 g compared with the full sample, suggesting the importance of considering heterogeneity. Japanese public policy for perinatal care may have contributed to this limited impact of birth weight. Additional care aimed at preventing low birth weight and greater focus on smaller babies would be more beneficial.