OBJECTIVES: Multiple births expose mothers and infants to disproportionate risks and incur substantial healthcare costs. We quantified the contribution of in vitro fertilization (IVF) to multiple births after universal national insurance coverage for IVF was introduced in South Korea. METHODS: This retrospective cohort study used the Korean National Health Insurance Service Mother-Child Linked Database and included 403,708 deliveries from October 2018 to December 2023. IVF exposure was identified from procedure claims, and plurality was determined from linked infant records. We estimated the adjusted relative risk (RR) of multiple births for IVF versus non-IVF deliveries and the population-attributable fraction (PAF) of multiple births attributable to IVF, overall and by age group. RESULTS: Of 403,708 deliveries, 65,442 (16.2%) involved IVF; this proportion increased from 9.4% in 2018 to 22.6% in 2023. Multiple births occurred in 21.9% of IVF deliveries and 1.9% of non-IVF deliveries. The overall adjusted RR for IVF was 8.67 (95% confidence interval [CI]: 8.40-8.94). Age-specific adjusted RRs were highest in women aged 25-29 years (12.37; 95% CI: 11.09-13.80) and lowest in those aged 40-49 years (7.43; 95% CI: 6.68-8.26). The overall PAF was 55.4%, driven largely by women aged 40-49 years (66.1%). In women aged 30-34 years, the PAF was 46.8%. CONCLUSION: IVF accounts for more than half of multiple births in South Korea, with a substantial contribution among women aged ≤35 years. These findings support policies that promote elective single embryo transfer.