Birth Weight-Dependent Regional Disparities in 28-Day and 1-Year Survival of Preterm Infants: Seoul Capital Area vs. Non-Capital Regions, South Korea, 2002–2021

<b>Background/Objectives</b>: Regional disparities in neonatal care capacity may have a disproportionate impact on the smallest and most vulnerable infants. In South Korea, where specialized perinatal resources are concentrated in the Seoul Capital Area (Seoul, Gyeonggi, and Incheon), it remains unclear how these disparities vary by birth weight and time since birth. <b>Methods</b>: We conducted a nationwide, population-based cohort study of preterm infants (<37 weeks gestation) born between 2002 and 2021 using the Korean National Health Insurance Service database. Residential address at birth classified infants into Seoul Capital Area or regions outside the Seoul Capital Area. We examined 28-day and one-year all-cause mortality using multivariable logistic regression, adjusting for sex, birth weight category, early transfer, medical aid status, maternal age, and antenatal visits. Birth weight-stratified analyses assessed effect modification. Major morbidities were evaluated with multivariable Cox models. <b>Results</b>: Among 204,245 preterm infants, those residing outside the Seoul Capital Area had higher adjusted odds of mortality at both 28 days (odds ratio 1.46; 95% confidence interval [CI], 1.30-1.64) and one year (odds ratio 1.25; 95% CI, 1.17-1.34) than those residing in the Seoul Capital Area. Disparities were minimal in infants ≥2500 g but increased progressively in lower birth weight strata, peaking among extremely low birth weight infants (<1000 g) (28-day odds ratio 1.67; 95% CI, 1.40-1.97; one-year odds ratio 1.54; 95% CI, 1.37-1.73). <b>Conclusions</b>: Regional survival disparities among preterm infants in South Korea widen with decreasing birth weight, underscoring the need for targeted neonatal care and post-discharge support in underserved regions.