BACKGROUND: Long continuous periods of working contribute to fatigue, which is an established risk factor for adverse patient outcomes in many clinical specialties. The total number of hours worked by delivering clinicians before delivery therefore may be an important predictor of adverse maternal and neonatal outcomes. OBJECTIVE: We aimed to examine how rates of adverse delivery outcomes vary with the number of hours worked by the delivering clinician before delivery during both day and night shifts. STUDY DESIGN: We conducted a retrospective cohort study of 24,506 unscheduled deliveries at an obstetrics center in the United Kingdom from 2008-2013. We compared adverse outcomes between day shifts and night shifts using random-effects logistic regression to account for interoperator variability. Adverse outcomes were estimated blood loss of ≥1.5 L, arterial cord pH of ≤7.1, failed instrumental delivery, delayed neonatal respiration, severe perineal trauma, and any critical incident. Additive dynamic regression was used to examine the association between hours worked before delivery (up to 12 hours) and risk of adverse outcomes. Models were controlled for maternal age, maternal body mass index, parity, birthweight, gestation, obstetrician experience, and delivery type. RESULTS: We found no difference in the risk of any adverse outcome that was studied between day vs night shifts. Yet, risk of estimated blood loss of ≥1.5 L and arterial cord pH of ≤7.1 both varied by 30-40% within 12-hour shifts (P<.05). The highest risk of adverse outcomes occurred after 9-10 hours from the beginning of the shift for both day and night shifts. The risk of other adverse outcomes did not vary significantly by hours worked or by day vs night shift. CONCLUSION: Number of hours already worked before undertaking unscheduled deliveries significantly influences the risk of certain adverse outcomes. Our findings suggest that fatigue may play a role in increasing the risk of adverse delivery outcomes later in shifts and that obstetric work patterns could be better designed to minimize the risk of adverse delivery outcomes.