The purpose of this study was to examine the association between time of delivery and perinatal outcomes in singleton deliveries at Japanese Red Cross Katsushika Maternity Hospital. We performed a retrospective cohort study of all singleton deliveries at ≥22 weeks' gestation from 2002 through 2009 at our hospital. During the nighttime period (12 midnight to 8 am), although the rate of delivery of high-risk pregnancies was low, the rate of low umbilical artery pH (<7) was higher than that during the daytime periods (8 am to 4 pm) (odds ratio 1.9, 95% confidence interval 1.1-3.4, p=0.02). During the nighttime period, in addition, the rate of low umbilical artery pH in patients with emergent cesarean delivery was significantly higher than that during the daytime period (odds ratio 6.9; 95% confidence interval 2.5-19, p<0.01). In our hospital, the rate of adverse neonatal outcomes associated with cesarean birth was increased at nighttime period.