Multiple studies have concluded that induction of labor, especially among nulliparous women with an unfavorable cervix, increases the risk of cesarean delivery.1–8 The results of many of these studies, however, cannot be generalized to women undergoing elective induction because the majority of studies have included women with medical indications for delivery such as diabetes or preeclampsia. These medical indications may be independent risk factors for cesarean delivery.9
An additional issue is the selection of the control group, namely, women in spontaneous labor. Comparing women who undergo an induction of labor to women who spontaneously labor does not represent a true clinical scenario because clinicians cannot choose spontaneous labor. Rather, the appropriate comparison group is a cohort of women who are expectantly managed and who may experience spontaneous labor or ultimately may require induction of labor for a medical indication. This comparison group more closely mimics the control group used in prospective trials, which have not consistently shown an increase in the rate of cesarean delivery. For example, in the largest prospective randomized trial of induction of labor compared with expectant management at 41 weeks of gestation, a reduction in cesarean delivery was found among women in the induction group.10
To date, in the one retrospective study comparing induction of labor to expectant management among nulliparous women, the investigators found a decreased cesarean delivery rate among women who were induced.11 However, the majority of women in their induction group were induced for medical, not elective, indications. Furthermore, there was no evaluation of cervical status, which is particularly important given its association with cesarean delivery, and differences between the two groups related to selection bias could have led to the observed findings. The purpose of this present study was to estimate whether cesarean delivery was increased among women with an unfavorable cervix who were electively induced versus expectantly managed at term.
MATERIALS AND METHODS
We conducted a retrospective study of nulliparous women with an unfavorable cervix who were either electively induced or expectantly managed at term. Inclusion criteria were nulliparity, gestational age 39 0/7 weeks or more, a singleton vertex gestation, known cervical status at 38 0/7–38 6/7 weeks, and a modified Bishop score less than 5.11 Women undergoing an induction of labor for a medical indication diagnosed before 39 weeks of gestation were excluded (diabetes, chronic hypertension, Rh isoimmunization). Induction of labor for nonmedical indications before 39 0/7 weeks of gestation is not permitted at our institution. Institutional Review Board approval for the study was obtained from Northwestern University before the start of the study.
The induction cohort was assembled using a perinatal database that identified all women who met inclusion criteria undergoing an induction of labor from July 1, 2006 to June 30, 2008. The duration of this study period was chosen to yield the appropriate sample size (as detailed with respect to the power calculation) based on our frequency of labor induction. To identify women who were being electively induced, each admission history and each physical was reviewed. Elective inductions were considered to occur either when the physician stated the indication of the induction was “elective” or when the stated indication did not meet accepted medical criteria for labor induction (eg, patient discomfort with pregnancy, patient's home far from hospital).13 Women induced at 41 0/7 weeks of gestation were not included in the induction cohort because induction of labor at or after this gestational age is acknowledged by the American College of Obstetricians and Gynecologists to constitute an obstetric indication.14 To avoid inclusion of women who began their induction of labor at 40 6/7 weeks of gestation but delivered at 41 0/7 weeks of gestation, only elective inductions of labor occurring between 39 0/7 and 40 5/7 weeks of gestation were included in the analysis. All women who underwent elective labor induction during the selected period of study were included. The examination on admission was used to calculate a modified Bishop score. An intracervical Foley catheter balloon was used for all cervical ripening.
A control group was then chosen from among all women meeting inclusion criteria who delivered during the same time period. Women were considered eligible for the control group if they had reached 39 0/7 weeks of gestation and were expectantly managed. These women, as they progressed in their gestation, could have experienced spontaneous labor, could have been induced for obstetric indications (eg, oligohydramnios, preeclampsia, postterm pregnancy), or could have undergone an unlabored cesarean delivery for an obstetric indication (eg, nonreassuring fetal status on antepartum testing). To limit the possibility that different practice styles could confound the results, women considered eligible for the control group had to be cared for by physicians who also had patients who were electively induced. Of all women who were expectantly managed and met criteria, 102 were randomly selected for this group using a computer-generated random number table. Cervical examination data between 38 0/7 and 38 6/7 weeks of gestation were obtained for all expectantly managed patients by reviewing office medical records.
Maternal and neonatal outcomes were recorded, as well as data regarding the obstetric course (eg, time at delivery, length of labor). Length of labor was defined as the time from admission to the labor and delivery unit to the time of delivery. The primary outcome was cesarean delivery. Given an 80% power and a two-sided α of .05, and to detect an increase in the cesarean delivery rate from 20% in the expectant management group to 40% in the elective induction of labor group, 182 women (divided equally between the two groups) were required for analysis. The Student t test or Mann-Whitney U test, as appropriate, was used to compare continuous variables, and the χ2 or Fisher exact test was used to compare proportions between groups. All tests were two-tailed and P<.05 was considered significant. Minitab 13 (College Station, TX) was used for analysis.
One hundred two women who underwent elective induction of labor were compared with 102 women who underwent expectant management during the period of study. The elective induction of labor and expectant management groups were similar with respect to demographic data (Table 1), except that women in the expectant management group were more likely to be delivered at a later gestational age (40 4/7 weeks compared with 40 0/7 weeks, P<.001).
Of women who were expectantly managed, 25.5% (26/102) delivered between 39 0/7 and 39 6/7 weeks of gestation, 39.2% (40/102) delivered between 40 0/7 and 40 6/7 weeks of gestation, and 35.3% (36/102) delivered between 41 0/7 and 42 0/7 weeks of gestation. Labor induction was ultimately required in 35.3% of the women, with the most common reasons being postdates (76.5%) and oligohydramnios (14.7%). There were no expectantly managed women who required a cesarean delivery without labor or who were delivered with an intrauterine fetal demise.
Labor characteristics were significantly different between women who were electively induced and those were expectantly managed (Table 2). Women who were electively induced spent more time in labor and delivery, whether measured as a continuous or categorical variable, and, not unexpectedly, received oxytocin more frequently (100% compared with 92%, P=.001). Electively induced women also were significantly more likely to deliver during daytime hours (6:00 am–6:00 pm). Conversely, there were no differences with regard to maternal outcomes (Table 3). Specifically, the rate of cesarean delivery was not different between the two groups. Furthermore, there were no differences with regard to other obstetric events (eg, operative vaginal delivery) or complications. Neonatal outcomes were also similar between groups with the exception of a decreased incidence of meconium-stained amniotic fluid among women who were induced (Table 4).
Last, to estimate whether elective inductions performed at a later gestational age (eg, at 40 3/7 weeks) could be responsible for affecting the observed outcomes, the analysis was performed again with the population of electively induced women restricted to those whose elective induction occurred before 40 0/7 weeks of gestation. Of the 102 women who underwent elective induction, 40 were between 39 0/7 and 39 6/7 weeks of gestation. When these women were compared with those who were expectantly managed after 39 0/7 weeks of gestation, the associations of induction with selected outcomes were unchanged. Women electively induced again were noted to have an increased mean length of labor (15.9 hours, interquartile range 13.2–19.6 compared with 12.7 hours, interquartile range 7.1–18.7, P=.001) and chance of labor being more than 12 hours (85% compared with 51%, P<.001), but no difference in frequency of cesarean delivery (35% compared with 34.3%, P=.94) than those who were expectantly managed.
This study was performed to compare outcomes among women with an unfavorable cervix who were electively induced with those of women who were expectantly managed. Other than a lower frequency of meconium among women who were electively induced, there were no significant differences in health outcomes, although there were significant differences in resource utilization on labor and delivery.
The findings of this study contrast with those of multiple previous studies that have shown at least a doubling of the cesarean delivery rate when labor is induced with an unfavorable cervix. For example, in their study of 2,200 women at term, Vaharatian et al2 reported an increase in the cesarean delivery rate from 13.9% among women in spontaneous labor to 41.3% among electively induced women who required cervical ripening. However, data from a study with this design can be used to compare outcomes of women who spontaneously labor to those who are induced, but not to compare the strategy of labor induction with its alternative. Immediate spontaneous labor is not the actual clinical alternative to induction of labor. Rather, the clinician's option for a patient at term without complications is expectant management, and the outcome may be either spontaneous labor or scheduled delivery (either by induction of labor or cesarean delivery, depending on the circumstances) at a later date for medical indications. The only other observational study comparing induction of labor to expectant management actually found a decreased rate of cesarean delivery among nulliparous women who were induced.11 Limitations of this study were the inclusion of women induced for medical indications and inability to control for cervical status, both of which could be confounding variables. In this present study, we were able to account for both these factors.
Although the findings of this study may seem contrary to popularly held beliefs regarding the consequences of labor induction, particularly in the context of an unfavorable cervix, plausible explanations exist for why induction of labor as opposed to watchful waiting may not increase the risk of cesarean delivery among women with an unfavorable cervix. Women with an unfavorable cervix at 39 weeks of gestation are the least likely to experience spontaneous labor and thus are more likely to ultimately require an induction for postterm pregnancy or other medical indications that arise while pregnant.15 Several studies have found a higher Bishop Score to be associated with an increased likelihood and sooner onset of spontaneous labor.16,17
There are several strengths of this study. An a priori power calculation was used to obtain an appropriate sample size to detect an effect size consistent with that of previous studies and thus decreases the possibility of a type II error. Furthermore, each chart was individually reviewed to ensure that patients were correctly classified as having undergone an induction that was purely elective. Finally, this study accounts for cervical status, which is known to be one of the most important predictors of labor induction outcome.
Nevertheless, there are several limitations. First, this study is retrospective and observational, and thus has potential for selection or confounding bias. Although the sample size was sufficient to detect differences in the primary outcome that were similar to differences noted in previous studies, it is possible that a smaller yet clinically relevant difference could not be detected. Second, this study did not have adequate power to detect differences in maternal or neonatal outcomes that occur less frequently than cesarean delivery, and these outcomes could be clinically relevant to the decision of whether to proceed with induction of labor. Finally, with 19 different provider groups performing deliveries at our institution, a variety of practice styles is present. It is possible that the association between induction and cesarean delivery, if it did exist, could be obscured by selection bias related to different management styles by different provider groups. We attempted to control for this possibility by excluding patients from the expectantly managed group who were cared for by providers other than those who performed elective inductions.
The findings of the present study should not be viewed as an endorsement of routine induction of labor. Induction of labor is associated with a greater utilization of labor and delivery resources with no clear maternal benefit. Also, although there was a decreased incidence of meconium staining among women who were induced, this did not translate into any difference in actual health outcomes. In the present study, labor induction resulted in 30% longer time spent in labor and delivery. This is consistent with the findings of Seyb et al,7 who demonstrated a 33% increase in time spent in labor and delivery among women who underwent labor induction. This additional time translates into increased costs. Maslow et al3 found that labor induction resulted in a 25% increase in costs, even when comparing only inductions that resulted in vaginal delivery. Rather, these findings should be used to highlight the need for an appropriately powered randomized trial of induction of labor.
1. van Gemund N, Hardeman A, Scherjon SA, Kanhai HH. Intervention rates after elective induction of labor compared to labor with a spontaneous onset. A matched cohort study. Gynecol Obstet Invest 2003;56:133–8.
2. Vahratian A, Zhang J, Troendle JF. Labor progression and risk of cesarean delivery in electively induced nulliparas. Obstet Gynecol 2005;105:698–704.
3. Maslow AS, Sweeny AL. Elective induction of labor as a risk factor for cesarean delivery among low-risk women at term. Obstet Gynecol 2000;95:917–22.
4. Heffner LJ, Elkin E, Fretts RC. Impact of labor induction, gestational age, and maternal age on cesarean delivery rates. Obstet Gynecol 2003;102:287–93.
5. Dublin S, Lydon-Rochelle M, Kaplan RC, Watts DH, Critchlow CW. Maternal and neonatal outcomes after induction of labor without an identified indication. Am J Obstet Gynecol 2000;183:986–94.
6. Yeast JD, Jones A, Poskin M. Induction of labor and the relationship to cesarean delivery. A review of 7001 consecutive inductions. Am J Obstet Gynecol 1999;180:628–33.
7. Seyb ST, Berka RJ, Socol ML, Dooley SL. Risk of cesarean delivery with elective induction of labor at term in nulliparous women. Obstet Gynecol 1999;94:600–7.
8. Vrouenraets FP, Roumen FJ, Dehing CJ, van den Akker ES, Aarts MJ, Scheve EJ. Bishop score and risk of cesarean delivery after induction of labor in nulliparous women. Obstet Gynecol 2005;105:688–9.
9. Ennen CS, Bofill JA, Magann EF, Bass JD, Chauhan SP, Morrison JC. Risk factors for cesarean delivery in preterm, term and post-term patients undergoing induction of labor with an unfavorable cervix. Gynecol Obstet Invest 2009;67:113–7.
10. Hannah ME, Hannah WJ, Hellmann J, Hewson S, Milner R, Willan A. Induction of labor as compared with serial antenatal monitoring in post-term pregnancy. A randomized controlled trial. The Canadian Multicenter Post-term Pregnancy Trial Group. N Engl J Med 1992;326:1587–92.
11. Caughey AB, Nicholson JM, Cheng YW, Lyell DJ, Washington AE. Induction of labor and cesarean delivery by gestational age. Am J Obstet Gynecol 2006;195:700–5.
12. Lange AP, Secher NJ, Westergaard JG, Skovgård I. Prelabor evaluation of inducibility. Obstet Gynecol 1982;60:137–47.
13. ACOG Committee on Practice Bulletins. ACOG Practice Bulletin No. 107: Induction of labor. Obstet Gynecol 2009;114:386–97.
14. ACOG Committee on Practice Bulletins–Obstetrics. ACOG Practice Bulletin. Clinical management guidelines for obstetricians-gynecologists. Number 55, September 2004 (replaces practice pattern number 6, October 1997). Management of postterm pregnancy. Obstet Gynecol 2004;104:639–46.
15. Caughey AB, Musci TJ. ACOG Committee on Practice Bulletins–Obstetrics. Complications of term pregnancies beyond 37 weeks of gestation. Obstet Gynecol 2004;103:57–62.
16. Rozenberg P, Goffinet F, Hessabi M. Comparison of the Bishop score, ultrasonographically measured cervical length, and fetal fibronectin assay in predicting time until delivery and type of delivery at term. Am J Obstet Gynecol 2000;182(1 Pt 1):108–13.
17. Strobel E, Sladkevicius P, Rovas L, De Smet F, Karlsson ED, Valentin L. Bishop score and ultrasound assessment of the cervix for prediction of time to onset of labor and time to delivery in prolonged pregnancy. Ultrasound Obstet Gynecol 2006;28:298–30.