Induction of twin pregnancy and the risk of caesarean delivery: a cohort study

This study suggests that induction of twin pregnancies is associated with a two-fold increase in risk of caesarean section compared with spontaneous labour onset, and that the increase in risk is associated to the need for cervical ripening. Despite the increased risk, the majority had a vaginal delivery. In women where amniotomy was possible, induction of labour had no increased risk of caesarean delivery. Almost half of pregnancies were induced in this study; the majority had a medical indication and were therefore necessary despite the increased risk of caesarean section. However, approximately 20 % had an elective induction and, women who want induction when there is no medical reason should be counselled about the benefit of postponing induction until amniotomy is possible.

The study is limited by the retrospective design and thus prone to potential confounding. By reviewing all records and by detailed collection of data on maternal, labour and delivery characteristics, an attempt was made to control for confounders, but some may still have been unidentified. Given that a substantial number of women declined participation, the possibility of selection bias must be recognized. To control for changes in management over time, year of delivery was controlled for in the regression analysis. Examples of changes during the study period include: the caesarean section rate before labour has increased; and twin deliveries are planned from 38 instead of 40 weeks’ gestational age. Generalization of the results from the predominantly Scandinavian population to other nationalities may be limited.

This is the first study on twin pregnancies that reports on the risk for caesarean section by induction method. However, women with spontaneous labour onset were used as a control group when the actual alternative is expectant management [15]. Retrospective cohort studies on elective induction (i.e. without medical induction) of labour in a specific week, compared to expectant management in single gestations at term, have not found a difference in risk for caesarean section from 37 weeks gestational age [16‐20], this is consistent with results from randomized controlled trials [21, 22]. When induction of twin pregnancies has been compared with expectant management, no difference in caesarean section rates have been found, albeit the sample sizes were too small to address the outcome in these studies [14, 23]. Results from singleton pregnancies may not be directly translated to twin pregnancies and, to date, studies large enough to compare expectant management with induction of labour in twin gestations are lacking.

Data on induction of twin pregnancies are limited in the literature and information on the risk of caesarean section related to induction methods is sparse. In a retrospective observational study of induced twin pregnancies, a non-significant higher caesarean delivery rate with vaginally administered misoprostol (n = 57), compared with oxytocin (n = 77), was found [13]. The authors of that study speculated that the higher caesarean section rate was due to the higher rate of nulliparity and unripe cervical status in the misoprostol group. The present study confirms that nulliparity, as well as unripe cervical status, are independent risk factors for caesarean section in induced twin deliveries. In one study of twin gestations, in which outcomes of induced labours (n = 36) were compared with expectant management (n = 45), oxytocin was used in half of induced labours and Foley catheter or vaginal application of Prostaglandin E₂ in the other half [14]. There was no difference in caesarean section rates; however, in that study, inductions were not stratified by method used. Another small study of 36 twin pregnancies, randomized to expectant management or induction with oral prostaglandin E₂ at 37 gestational weeks, found a non–significant lower rate of caesarean deliveries in the induction group [23]. There were significantly more women with pre labour rupture of membranes in the expectant management group, indicating that the randomization did not result in comparable groups in that study, and maternal infection was the most common indication for caesarean section.

Failed induction, with a caesarean section performed in the latency phase, was a common indication for caesarean section (31 %) in the present study. Studies on the latency phase in twin gestations are sparse and contradictory. One study reports the latent phase to be shorter in twin gestations compared with singletons [24], unlike the finding in a recent cohort study, where twin gestations had significantly less cervical dilation at admission [1]. In singleton gestations, there is data to suggest that latent phase duration is longer in induced compared with spontaneous labour, whereas duration of the active phase is similar [25]. An arrest diagnosis before 6 cm dilation in induced labour is thought better avoided [25] and, by allowing longer duration of the latent phase, unnecessary caesarean deliveries for failed induction can probably be avoided [26].

Twin gestations have slower labour progress compared to singletons, regardless of parity, and require 1–3 hours more to complete the first stage of labour compared with singletons, according to a study by Leftwich and colleagues [1]. The authors of the study propose that patience in providers and patients, concerning the time needed to come through the active phase of twin labours, may reduce caesarean deliveries for failure to progress. Sub analysis of induced labours was not done in that study, although approximately 40 % of labours were induced in both groups and, information on induction method was not described [1]. The results of the present study indicate that when induced labour reaches the active phase, the progress is comparable to those with a spontaneous labour onset. In support of this finding, no difference in mean labour duration was found in previous studies in which induction of twin gestations was compared to expectant management [14, 23].

Twin gestation is associated with an increased risk of morbidity to the neonate. Caesarean section before labour is believed to reduce this risk and is one reason for the increase in caesarean delivery rate seen worldwide [27, 28]. In a recently published randomized controlled trial including twins between 32 and 38 weeks, caesarean section before labour did not reduce the risk of adverse neonatal or maternal outcomes compared with planned vaginal delivery [29]. In contrast, evidence from observational studies suggest there may be some benefit of pre-labour caesarean section for neonates at 36 weeks gestational age and greater, although the absolute rate of adverse neonatal outcome was reported low [30‐32]. For the woman, a scarred uterus is one important source of morbidity and increases the risk of caesarean section in subsequent deliveries [33, 34]. Given current evidence, a trial of labour in twin pregnancy is still considered a reasonable and safe option [29]. In case of induction, the present study might provide information that could be useful in the counselling of women with a twin gestation.