AbstractBackgroundSevere maternal morbidity is an indicator for quality of maternal care. Recently, there has been growing interest in identifying which provision factors affect the quality of maternity care. The extent to off-hour delivery on SMM rates contributes to individual or provision factor in Korea has not been studied. This study aimed to determine the relationship between off-hour delivery and SMM during childbirth hospitalization.MethodsThis is a population-based retrospective cohort study. Data were extracted from the Korean National Health Insurance Service-National Sample Cohort for 90 072 cases of delivery in Korea between 2003 and 2013. The main outcome was SMM which was determined using the Center for Disease Control and Prevention’s algorithm. A generalized estimating equation model with log link was performed for the relationship with SMM and day/time of delivery adjusted covariates.ResultsOf the 90 072 delivery cases, 2085 (2.31%) had SMM. Women who were on weekdays at night time or on weekend delivery had a higher risk of SMM compared with those who were on weekdays at daytime (RR 1.26, 95% CI 1.10–1.46, and RR 1.58, 95% CI 1.30–1.93, respectively).ConclusionWeekday at night time or weekend delivery was related to the risk of SMM. Policymakers should provide financial support and systematically allocate adequate human resources and labour facilities in vulnerable areas, as well as during weekends and night times to improve the quality of intrapartum and postpartum maternity care.