IMPORTANCE: While various policies to support couples experiencing infertility have been introduced due to the fertility rate rapidly dropping in developed countries, few large-scale nationwide cohort studies have evaluated the outcomes of assisted reproductive technology (ART) health insurance coverage policies. OBJECTIVE: To evaluate ART health insurance coverage for multiple pregnancies and births in Korea. DESIGN, SETTING, AND PARTICIPANTS: This population-based cohort study used delivery cohort data from the Korean National Health Insurance Service database between July 1, 2015, and December 31, 2019. A total of 1 474 484 women were included after exclusion of those who gave birth at nonmedical institutions and those with missing data. EXPOSURE: Two 27-month periods were examined before and after the Korean National Health Insurance Service had begun covering ART treatment (preintervention period, July 1, 2015, to September 30, 2017; postintervention period, October 1, 2017, to December 31, 2019). MAIN OUTCOMES AND MEASURES: Multiple pregnancies and multiple births were identified by International Statistical Classification of Diseases and Related Health Problems, Tenth Revision diagnosis codes. Total births were defined as the total number of babies born to each pregnant woman during the follow-up period. An interrupted time series with segmented regression was conducted to analyze the time trend and its change in outcomes. Data analysis was conducted between December 2, 2022, and February 15, 2023. RESULTS: Of the 1 474 484 women eligible for the analysis (mean [SD] age, 33.2 [4.6] years), approximately 1.60% had multiple pregnancies and 1.10% had multiple births. After covering ART treatment, the likelihood of multiple pregnancies and multiple births was estimated to increase by 0.7% (estimate, 1.007; 95% CI, 1.004-1.011; P < .001) and 1.2% (estimate, 1.012; 95% CI, 1.007-1.016; P < .001) compared with before coverage. The probability of an increase in the number of total births per pregnant woman after the intervention was estimated to be 0.5% (estimate, 1.005; 95% CI, 1.005-1.005; P < .001). The relatively high-income class above the median income showed a decreasing trend in multiple births and total births before the intervention, but after the intervention, a significant increase was observed. CONCLUSIONS AND RELEVANCE: This population-based cohort study found that the possibility of multiple pregnancies and births in Korea significantly increased after the implementation of an ART health insurance coverage policy. These findings suggest that the development and coverage of policies to support couples experiencing infertility may help address low fertility rates.