The number and rate of twin births continues to rise, from 2.2% of all U.S. live births in 1990 to 3.2% of all U.S. live births in 2005.1 In the United States, most twins are delivered by cesarean, and this rate has been increasing. From 1995 to 2008, the cesarean delivery rate for twin births in the United States rose from 53.4% to 75.0%.2 This increase was seen in both vertex and breech twins and after adjustment for risk, there was an average increase in the cesarean delivery rate of 5% per year.2
Induction of labor is significantly associated with cesarean delivery in singleton pregnancies, particularly women who are nulliparous and women with an unfavorable cervix.3–7 However, in regard to twin pregnancies, there are far less published data regarding outcomes of induced labors. Published studies typically combine induced labors and spontaneous labors that require augmentation8,9 or do not include an adequate control group of singleton pregnancies.10–13 Although studies have compared progress of labor in twin compared with singleton pregnancies, they either excluded patients with induced labors14 or only compared the active phase of labor in patients after 5 cm cervical dilatation.15 Therefore, the published data specifically regarding outcomes after induction of labor in twin pregnancies compared with singleton pregnancies are limited.
Because twin pregnancies are at increased risk for complications that warrant early delivery such as preeclampsia and fetal growth restriction and twin pregnancies, even when dichorionic and uncomplicated, typically undergo elective delivery at 38 weeks of gestation,16 many patients with twin pregnancies who are not in labor will have an indication for delivery and their doctors will be faced with the decision whether to recommend induction of labor or elective cesarean delivery. To best counsel these patients, one would need to know the likelihood of a vaginal delivery after induction of labor. Based on this likelihood, the doctor and patient can decide whether induction of labor is worthwhile as compared with an elective cesarean delivery. However, without data specific to twins, the only data to rely on would be from singleton pregnancies, which may not be applicable. Therefore, the purpose of this study was to estimate the likelihood of cesarean delivery and length of labor in twin pregnancies undergoing induction of labor as compared with singleton pregnancies.
PATIENTS AND METHODS
After Mount Sinai School of Medicine institutional review board approval was obtained, a retrospective cohort study was performed by reviewing the charts of all patients with twin pregnancies delivered by one maternal–fetal medicine practice between June 2005 and January 2012. Inclusion criteria were any patient with a twin pregnancy greater than 24 weeks of gestation with intact membranes who underwent induction of labor. Patients who presented in latent or active labor, or with premature rupture of membranes, who were given augmentation of labor were not included. We also excluded patients who presented with contractions and some degree of cervical dilation who were augmented or induced for this indication. Therefore, our study cohort consists of patients with a twin pregnancy who underwent true induction of labor. As a comparison group, we randomly selected in a 1:1 ratio patients from the same maternal–fetal medicine practice with a singleton pregnancy who underwent induction of labor in the same time period. The same exclusion criteria were applied to the singleton pregnancies such that the control group of patients with singleton pregnancies was appropriate.
The antepartum, induction, and intrapartum care of all patients in the twin and singleton groups were managed by the same six obstetricians over the study period (N.F., C.K., A.R., and D.S. as well as two nonauthors). Our methods for induction of labor do not differ between twin and singleton pregnancies. For patients who require cervical ripening, we either use 25 micrograms of vaginal misoprostol every 3 to 4 hours or introduce a transcervical Foley catheter with a 60-mL balloon, which is applied to gentle traction. Once the cervix is dilated, amniotomy is performed and intravenous oxytocin is administered toward a goal of contractions approximately every 3 minutes. For patients with a favorable cervical examination, amniotomy is performed with or without the administration of intravenous oxytocin as necessary. Our indications for cesarean delivery in labor do not differ between twin and singleton pregnancies.
Our management of labor in twin pregnancies has been described previously.17 In short, contraindications to vaginal delivery of twins in our practice are nonvertex presenting twin, nonvertex second twin with an estimated fetal weight greater than 20% larger than the presenting twin, nonvertex second twin with an estimated fetal weight less than 1,500 g, and other usual contraindications to labor (eg, placenta previa, prior classical cesarean delivery). Patients with twin pregnancies have regional anesthesia in labor. If indicated, we do perform breech extraction of the second twin, sometimes after internal version.
Baseline characteristics including parity, indication for delivery, cervical dilation, and effacement on initiation of induction were recorded. We categorized indications for induction into four groups: 1) gestational age (40 weeks or greater for singleton pregnancies, 38 weeks or greater for dichorionic twin pregnancies, 37 weeks or greater for monochorionic twin pregnancies), 2) maternal (eg, hypertension, diabetes, other maternal disease), 3) fetal (eg, fetal growth restriction, oligohydramnios, intrahepatic cholestasis of pregnancy), and 4) other. Indications for cesarean delivery were categorized into two groups: 1) arrest disorders (failed induction, arrest of labor in any stage of labor), and 2) fetal indications (nonreassuring fetal heart rate, umbilical cord prolapse).
Our primary outcome was mode of delivery (vaginal or cesarean). We also compared the length of labor (defined as the time from initiation of induction to the time of delivery in patients who had a vaginal delivery) as well as indications for cesarean delivery. Assuming a cesarean delivery rate of approximately 20% in patients with singleton pregnancies undergoing induction of labor,3,4 to have 80% power to detect an increased rate of cesarean delivery in twin pregnancies to 40% with an α error of 5%, 91 patients would be needed in each group. Chi square, Mann-Whitney U, and Student's t test were used when appropriate. We compared outcomes in all patients as well as subgroup analyses in nulliparous patients only and multiparous patients only. A planned multivariable stepwise regression analysis was also performed to control for differences in baseline characteristics between the groups. A P value of ≤.05 was considered significant.
Over the course of the study period, 505 patients with twin pregnancies greater than 24 weeks of gestation were delivered in our practice. Of them, 286 underwent a planned cesarean delivery (either before labor or on presentation in labor), and 119 attempted vaginal delivery after spontaneous labor or rupture of membranes, leaving 100 patients (19.8%) with twin pregnancy who underwent induction of labor and met the inclusion and exclusion criteria. The control group was comprised of a random sample of 100 patients delivered by our practice with singleton pregnancies who underwent induction of labor over the same time period. Baseline characteristics in the two groups are shown in Table 1. Maternal age, prepregnancy body mass index (calculated as weight (kg)/[height (m)]2), parity, cervical dilation, effacement, and epidural use were similar between the two groups. As expected, the gestational age at delivery was significantly earlier in the twin group.
Delivery outcomes are shown in Table 2. There were no patients in the twin group with a combined vaginal–cesarean delivery. There was no difference in the likelihood of cesarean delivery between patients with twin and singleton pregnancies (19% in twin pregnancies compared with 21% in singleton pregnancies, P=.724) nor was there a difference in the median or mean length of labor or the indication for cesarean delivery between the groups. When we separated by parity, we found similar results. In nulliparous patients only as well as in multiparous patients only, patients with twin pregnancies and induced labor were no more likely to have a cesarean delivery than patients with singleton pregnancies and induced labor. There were also no differences in the median or mean length of labor or the indication for cesarean delivery.
A multivariable regression analysis was done to control for baseline characteristics and assess for factors that are independently associated with cesarean delivery in patients undergoing induction of labor and the results are shown in Table 3. On adjusted analysis, twin pregnancy was not independently associated with cesarean delivery (odds ratio [OR] 1.791, 95% confidence interval [CI] 0.509–6.333, P=.364). The factors independently associated with cesarean delivery were maternal age, which was positively associated with cesarean delivery, and prior vaginal delivery and cervical dilation, which were both negatively associated with cesarean delivery. The regression analysis was repeated in a stepwise fashion only including these three significant factors as well as twin compared with singleton pregnancy and the results did not differ. Twin pregnancy was not associated with cesarean delivery (OR 0.901, 95% CI 0.391–2.078, P=.01). Age remained positively associated with cesarean delivery (OR 1.081, 95% CI 1.019–1.147, P=.010), prior vaginal delivery remained negatively associated with cesarean delivery (OR 0.114, 95% CI 0.024–0.529, P=.006), and cervical dilation remained negatively associated with cesarean delivery (OR 0.457, 95% CI 0.298–0.700, P<.001).
In this study, we found no difference in the rate of cesarean delivery, length of labor, and indications for cesarean delivery between women with twin pregnancies undergoing induction of labor and patients with singleton pregnancies undergoing induction of labor. This indicates that although twin pregnancies have an increased risk of cesarean delivery overall, it does not appear to be the result of labor dysfunction or failed induction. This is reassuring for patients with twin pregnancies who desire a vaginal delivery who are advised that delivery is indicated and induction of labor is an option. These patients can be reassured that their likelihood of a successful induction of labor is not reduced as a result of their twin pregnancy specifically, and they can be counseled similar to a patient with a singleton pregnancy. Prior studies do not address this question specifically. One study demonstrated that twin labors actually progress faster than singleton labors,14 which is also reassuring for patients with twin pregnancies who desire a vaginal delivery. We have previously reported a low (15.4%) cesarean delivery rate for twins in labor (spontaneous or induced).17 These findings indicate that the increased cesarean delivery rate seen in twin pregnancies is primarily the result of an increased rate of planned cesarean deliveries before labor and not the result of abnormal or nonreassuring labors.
On adjusted analysis, twin pregnancy was not an independent risk factor for cesarean delivery. We found that among patients undergoing induction of labor, the independent risk factors for cesarean delivery were maternal age, which increased the risk of cesarean delivery, and prior vaginal delivery and cervical dilation, which both reduced the risk of cesarean delivery. These findings are consistent with prior studies in singleton pregnancies. Furthermore, although there are two fetuses being monitored in twin inductions, which one might think would increase the risk of cesarean delivery as a result of fetal indications, we found that the indication for cesarean delivery in twin pregnancies was no more likely to be the result of fetal indications.
Our methods for induction of labor in twin and singleton pregnancies include the use of misoprostol, transcervical Foley, amniotomy, and oxytocin, which are all standard methods for induction of labor in the United States. In our practice, we do not alter our induction methods as a result of twin pregnancy specifically nor do we have different indications for cesarean delivery in labor for twin pregnancies. Our interpretation of the progress of labor also is uniform for singleton and twin pregnancies. This uniform induction and labor management of all patients reduces the possible bias in our study, which is important in interpreting the results and applying them to other populations. If this study included all patients in our institution, there would be increased variance in regard to assessment of cervical dilation at baseline, management of labor, interpretation of the labor curve, and fetal heart rate pattern, which would reduce the validity of the results. However, the uniformity of management also introduces a potential weakness, that the results could be less generalizable to patients with doctors with different management styles or to different patient populations. For example, our experience with vaginal delivery of twin pregnancies and breech extraction of the second twin17 may lead to a less restrictive management of twin labor, and doctors who are not as comfortable with twin deliveries may be more likely to diagnose an arrest disorder and perform a cesarean delivery in labor in twin pregnancies. Additionally, our population was primarily white and all patients had private health insurance. Therefore, our study should be repeated in other populations. However, our findings indicate that in certain settings, patients with twin pregnancies can be reassured that induction of labor does not increase the risk of cesarean delivery any more so than in a singleton pregnancy.
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