AbstractObjectiveTo study the busy day effect on selected neonatal adverse outcomes in different sized delivery hospitals and in the entire nationwide obstetric ecosystem.DesignA cross‐sectional register study.SettingThe lowest and highest 10% of the daily delivery volume distribution were defined as quiet and busy days, respectively. The days between (80%) were defined as optimal delivery volume days. The differences in the incidence of selected adverse neonatal outcome measures were analysed between busy versus optimal days and quiet versus optimal days at the hospital category and for the entire obstetric ecosystem level.PopulationA total of 601 247 singleton hospital deliveries between 2006 and 2016, occurred in non‐tertiary (C1–C4, stratified by size) and tertiary level (C5) delivery hospitals.MethodsAnalyses were performed by the methods of the regression analyses with crude and adjusted odds ratios including 99% CI.Main outcome measuresBirth asphyxia.ResultsAt the ecosystem level, adjusted odds ratio for birth asphyxia was 0.81 (99% CI 0.76–0.87) on busy versus optimal days. Breakdown to hospital categories show that adjusted odds ratios for asphyxia on busy versus optimal days in non‐tertiary hospitals (C3, C4) were 0.25 (99% CI 0.16–0.41) and 0.17 (99% CI 0.13–0.22), respectively, and in tertiary hospitals was 1.20 (99% CI 1.10–1.32).ConclusionsBusy day effect as a stress test caused no extra cases of neonatal adverse outcomes at the ecosystem level. However, in non‐tertiary hospitals busy days were associated with a lower and in tertiary hospitals a higher incidence of neonatal adverse outcomes.