We estimated that we would have adequate statistical power AUY922 price to detect important differences in postpartum haemorrrhage, third and fourth degree tears and neonatal unit admission.6 A sample size of 600 deliveries could detect an OR of 2.25 with 80% power and 5% significance level assuming a complication rate of 5% in the lower risk group. Data analysis was performed with the statistical package SPSS (V.20.0). Results A total cohort of 597 nulliparous women consented for an OVD between February and November 2013. Of these, 9 women (1.5%) proceeded to a spontaneous vaginal delivery and 22 (3.7%) delivered by CS. The cohort was evenly divided between delivery by day (n=301; 50.4%) and

at night (n=296; 49.6%). The peak times for OVD were 18:00–20:00 and 23:00–00:00, and the quietest time periods were 03:00–04:00 and 08:00–10:00 (figure 1). Maternal and neonatal characteristics are presented in table 1. Women with pre-eclampsia were less likely to deliver by day than at night, OR 0.29 (95% CI 0.09 to 0.91) and low birthweight babies (<2.5 kg) were more likely to deliver by day, OR 5.58 (95%

CI 1.23 to 25.38). The maternal and neonatal characteristics of the cohort were otherwise similar in relation to time of birth. Labour characteristics and indication for OVD were similar except for induction of labour where women delivered more frequently at night (43% vs 56%; OR 0.59 (95% CI 0.43 to 0.81) for daytime delivery; table 2). Table 1 Maternal and neonatal characteristics in relation to time of operative vaginal delivery Table 2 Labour characteristics in relation to time of

operative vaginal delivery Figure 1 Operative vaginal deliveries performed throughout the 24-hour time period. The primary instrument of choice for all OVDs was the Kiwi disposable vacuum (64.8%) followed by non-rotational forceps (26.5%) (table 3). More than half the deliveries were mid-station at each time period and similar proportions required rotation for a malposition. The grade of operator varied by time of birth with a higher proportion of OVDs performed by mid-grade operators at night (37.9% vs 50.4%; OR 0.60 (95% CI 0.43 to 0.83) for daytime delivery). A second operator was more likely to be involved during the day, Cilengitide OR 2.84 (95% CI 1.24 to 6.48), as was a supervising consultant, OR 2.26 (1.05 to 4.85). There were no significant differences between the incidence of sequential use of instruments, CS after assessment for OVD, or CS after a failed attempt at OVD. The mean time taken to complete the delivery was similar by day and at night (decision to delivery intervals 12.0 and 12.6 min, respectively). Table 3 Procedural factors in relation to time of operative vaginal delivery The maternal and neonatal morbidity outcomes are presented in table 4. The incidence of shoulder dystocia was higher by day than at night, adjusted OR 2.57 (1.05 to 6.