This study evaluated the association between off-hour deliveries and the risk of severe maternal morbidity (SMM). Data regarding Korean deliveries between 2005 and 2019 obtained from the National Health Insurance Service were used. SMM was evaluated using an algorithm developed by the United States Centers for Disease Control and Prevention. Modified Poisson regression analyses were conducted to investigate the association between off-hour deliveries and SMM, with stratification by hospital region and the number of beds. Approximately 32.7% of the 3,076,448 nulliparous women in this study delivered during off-hours, including 2.6% who experienced SMM. Patients who delivered at night had the highest risk of SMM (weekday nights, adjusted risk ratio (aRR): 1.41, 95% confidence interval (CI): 1.38-1.44; weekend nights, OR: 1.40, 95% CI: 1.34-1.46). The SMM of night deliveries was higher at hospitals located in small cities and those with 100-499 beds (weekend night: small cities, aRR: 1.49, 95% CI: 1.40-1.59; 100-499 beds, aRR: 1.83, 95% CI: 1.67-2.01; weekday night: small cities, aRR: 1.46, 95% CI: 1.42-1.51; 100-499 beds, aRR: 1.70, 95% CI: 1.62-1.79). Therefore, nighttime deliveries are associated with a higher risk of SMM, especially at hospitals located in small cities and those with 100-499 beds.
Keywords: maternal health; nighttime delivery; off-hour delivery; severe maternal morbidity.