Age-Period-Cohort Patterns in the Increase in Cesarean Section Rates in South Korea, 2007-2024: A Nationwide Population-Based Study.

BACKGROUND: Cesarean section (CS) rates are increasing rapidly worldwide, and the CS rate in South Korea reached 67.4% in 2024. Although many studies have examined these trends, few have assessed whether they reflect differences across maternal birth cohorts. We examined variation in the CS rate by maternal age, calendar period, and maternal birth cohort. METHODS: Using nationwide National Health Insurance claims data, we identified 6,635,390 deliveries among women aged 15-44 years in 2007-2024. The outcome was mode of delivery, classified as CS or vaginal delivery. We fitted hierarchical age-period-cohort cross-classified random-effects models, with maternal age entered as fixed linear and quadratic terms and calendar period and birth cohort as random effects. The models were adjusted for parity, insurance type, and hospital type and fitted separately by parity. RESULTS: The crude CS rate increased from 36.4% in 2007 to 67.4% in 2024. Maternal age was strongly associated with CS, but period and cohort variation persisted after adjustment (period τ2 = 0.117; cohort τ2 = 0.020). Estimated period random effects increased from 2015 onward, and estimated cohort random effects were positive among women born between 1992 and 2002. Age, period, and cohort patterns were more pronounced among primiparous women. DISCUSSION: Korea's increasing CS rate appears to reflect not only maternal aging but also period patterns after 2015 and elevated CS rates among recent birth cohorts, especially among primiparous women. These findings suggest that changes in the obstetric care environment and cohort-specific delivery practices should be considered when interpreting recent national trends.