Objective To compare rates of vaginal delivery and adverse outcomes of instrumental delivery trials in obstetric theatre compared to primary emergency full dilatation Caesarean section Design Retrospective cohort study Setting University teaching hospital Population Women with singleton, non-anomalous, pregnancy undergoing instrumental delivery trial in obstetric theatre Methods Data was collected from consecutive cases during 2014 until 2018 using clinical records. Multivariate regression analysis was used comparing groups per first delivery attempt. Main Outcome Measures Primary outcome was completion of vaginal delivery between all methods of instrumental delivery. Secondary outcome was a composite of immediate perinatal adverse outcomes for instrumental delivery modes and primary full dilatation Caesarean section. Results From 971 deliveries analysed: ventouse delivery was significantly less likely to achieve vaginal delivery compared to Keilland’s forceps delivery (OR 0.42, 95%CI 0.22-0.79). Once confounding factors were adjusted for, adverse outcome rates were less frequent in the Keilland’s forceps group compared with primary full dilatation Caesarean section (OR 0.37, 95% CI: 0.16-0.81), however the receiver operating characteristic curve produced from this model demonstrated low predictive value (AUC 0.64). Conclusions Attempting instrumental delivery in delivery suite theatre, as an alternative to primary emergency full dilatation Caesarean section, is both reasonable and safe. Ventouse delivery in this situation may be associated with a higher chance of failure than other modes of instrumental delivery, thus making appropriate choice of delivery method of paramount importance according to each clinical situation. Funding None Keywords Caesarean section, Keilland’s forceps, ventouse, trial of instrumental delivery