Abstract: The search resulted in 2693 articles that we screened on title and abstract for eligibility.Thirteen studies were selected for full text assessment. We included three randomised trials involving 1150 women. Two trials compared the administration of prostaglandins in the morning versus the evening in women with an unfavourable cervix, and one trial compared induction of labour in the morning versus the evening in women with a favourable cervix and/or ruptured membranes with intravenous oxytocin. Because of the different mechanism, we have reported results for these two comparisons separately. In the two trials comparing prostaglandins in the morning versus the evening there were few clinically significant differences between study groups for maternal or neonatal outcomes. One study reported a statistically significant preference by women to start induction of labour with prostaglandins in the morning. In the trial examining induction of labour with intravenous oxytocin, the number of neonatal admissions was statistically significantly increased in the group of women that started induction in the morning. This finding was unexpected, and while the trial authors offered some possible explanations for this, it is important that any future trials examine neonatal outcomes. Taking into account women's preferences that favoured administration of prostaglandins in the morning, we conclude that caregivers should preferably consider administering prostaglandins in the morning. There is no strong evidence that induction of labour with intravenous oxytocin in the evening is more or less effective than induction in the morning. Consideration may be given to start induction of labour with oxytocin in the evening when indicated.

Plain-language summary: In human and animal studies however, spontaneous onset of labour is proven to have a circadian rhythm with preference for start in the evening. Moreover, when spontaneous labour starts in the evening, total duration of labour shortens and less obstetric interventions are needed. Based on these observations one might assume, that starting induction of labour in harmony with the circadian rhythm of natural birth is more beneficial. This review found three studies that were of high quality with a total of 1150 women randomly allocated to induction in the morning or the evening. One trial used intravenous oxytocin in women who had a dilated cervix or rupture of membranes and two trials used prostaglandins to induce labour. Prostaglandins are hormones used when the cervix is not ripe, intravenous oxytocin is mostly needed afterwards to really get labour started. Therefore, these two different methods, prostaglandins and intravenous oxytocin, rely on a different mechanism and were assessed separately. This review found no differences in effect between starting induction in morning or evening on outcomes for mother or child. The risk of a vaginal birth using instruments, or risk of a caesarian section and use of epidural anaesthesia did not clearly differ between groups. One study reported that women had a preference to start induction of labour with prostaglandins in the morning, and more women in the evening admission group did not like the interruptions to sleep that were associated with the induction protocol. This review, with only three studies with two different comparisons, concludes that induction of labour in the evening is as effective and safe as induction in the morning. However, given the preference of most women, administration of prostaglandins should preferably be done in the morning.