Influence of time of delivery on risk of adverse neonatal outcome in different size of delivery units: a retrospective cohort study in Finland.

Purpose: To evaluate the impact of time of birth on adverse neonatal outcome in singleton term hospital births. Materials and methods: Medical Birth Register Data in Finland from 2005 to 2009. Study population was all hospital births (n = 263,901), excluding multiple pregnancies, preterm births <37 weeks, major congenital anomalies or birth defects, and antepartum stillbirths. Main outcome measures were either 1-minute Apgar score 0-3, 5-minute Apgar score 0-6, or umbilical artery pH <7.00, and intrapartum and early neonatal mortality. We calculated risk ratios (ARRs) adjusted for maternal age and parity, and 95% confidence intervals (CIs) to indicate the probability of adverse neonatal outcome outside of office hours in normal vaginal delivery, in vaginal breech delivery, in instrumental vaginal delivery, and in elective and nonelective cesarean sections. We analyzed different size-categories of maternity hospitals and different on-call arrangements. Results: Instrumental vaginal delivery had increased risk for mortality (ARR 3.31, 95%CI; 1.01-10.82) outside office hours. Regardless of hospital volume and on-call arrangement, the risk for low Apgar score or low umbilical artery pH was higher outside office hours (ARR 1.23, 95%CI; 1.15-1.30). Intrapartum and early neonatal mortality increased only in large, nonuniversity hospitals outside office hours (ARR 1.51, 95%CI; 1.07-2.14). Conclusions: Compared to office hours, babies born outside office hours are in higher risk for adverse outcome. Demonstration of more detailed circadian effects on adverse neonatal outcomes in different subgroups requires larger data.