Association between maternal risk factors and preterm birth in South Korea: a nationwide cohort study of 795,715 pregnancies.

backgroundPreterm birth (PTB), which is defined as delivery before 37 weeks of gestation, is the leading cause of neonatal morbidity, long-term developmental impairment, and infant mortality. In South Korea, PTB has become a critical concern amid declining fertility, delayed childbearing, and an increased reliance on assisted reproductive technology (ART). Comprehensive population-based evidence of contemporary maternal and healthcare-related risk factors is limited.

methodsWe conducted a retrospective cohort study using Health Insurance Review & Assessment Service claims data for all singleton live births between 2018 and 2022 (N = 795,715). Cox proportional hazards model with gestational age as the time axis were used to estimate the adjusted hazard ratios (aHRs) and 95% confidence intervals (CIs). Preterm birth is inherently a time-to-event process, where risk evolves dynamically over gestational age, and key exposures (e.g., pregnancy complications) arise during follow-up. Cox proportional hazards modeling with gestational age as the time axis is therefore epidemiologically justified and widely used in obstetric research. Pregnancy complications were modelled as time-varying covariates, and baseline hazards were stratified according to the delivery facility level.

resultsSignificant risk factors of PTB included a history of PTB (aHR 4.05, 95% CI 2.57–6.36), adolescent pregnancy (< 20 years; aHR 2.35, 95% CI 1.26–3.44), severe pregnancy complications (aHR 2.21, 95% CI 2.04–2.38), ART conception (aHR 1.32, 95% CI 1.18–1.47), and a history of miscarriage (aHR 1.34, 95% CI 1.21–1.48). Women covered by Medical Aid, reflecting a lower socioeconomic status, were also at an increased risk (aHR 1.65, 95% CI 1.06–2.57). Although the established model demonstrated excellent discrimination, False labor, while strongly associated with preterm birth, represents a clinically proximate predictive marker rather than an etiological cause, reflecting imminent risk as pregnancy progresses.

conclusionsThis nationwide analysis identified significant associations between recurrent obstetric history, adolescent pregnancy, ART, socioeconomic disadvantage, and the risk of PTB. These findings underscore the importance of early antenatal risk stratification, targeted support for vulnerable populations, and implementation of policies to address the structural determinants of maternal health. Insights from the demographic and healthcare context of Korea may inform global strategies to reduce PTBs.