Systematic Review: Elective Induction of Labor Versus Expectant Management of Pregnancy

Aaron B. Caughey; Vandana Sundaram; Anjali J. Kaimal; Allison Gienger; Yvonne W. Cheng; Kathryn M. McDonald; Brian L. Shaffer; Douglas K. Owens; Dena M. Bravata

doi:10.7326/0003-4819-151-4-200908180-00007

Reviews |18 August 2009

Systematic Review: Elective Induction of Labor Versus Expectant Management of Pregnancy Free

Aaron B. Caughey, MD, MPP, MPH, PhD; Vandana Sundaram, MPH; Anjali J. Kaimal, MD; Allison Gienger, BA; Yvonne W. Cheng, MD, MPH; Kathryn M. McDonald, MM; Brian L. Shaffer, MD; Douglas K. Owens, MD, MS; Dena M. Bravata, MD, MS

Article, Author, and Disclosure Information

Abstract

Background:

The rates of induction of labor and elective induction of labor are increasing. Whether elective induction of labor improves outcomes or simply leads to greater complications and health care costs is commonly debated in the literature.

Purpose:

To compare the benefits and harms of elective induction of labor and expectant management of pregnancy.

Data Sources:

MEDLINE (through February 2009), Web of Science, CINAHL, Cochrane Central Register of Controlled Trials (through March 2009), bibliographies of included studies, and previous systematic reviews.

Study Selection:

Experimental and observational studies of elective induction of labor reported in English.

Data Extraction:

Two authors abstracted study design; patient characteristics; quality criteria; and outcomes, including cesarean delivery and maternal and neonatal morbidity.

Data Synthesis:

Of 6117 potentially relevant articles, 36 met inclusion criteria: 11 randomized, controlled trials (RCTs) and 25 observational studies. Overall, expectant management of pregnancy was associated with a higher odds ratio (OR) of cesarean delivery than was elective induction of labor (OR, 1.22 [95% CI, 1.07 to 1.39]; absolute risk difference, 1.9 percentage points [CI, 0.2 to 3.7 percentage points]) in 9 RCTs. Women at or beyond 41 completed weeks of gestation who were managed expectantly had a higher risk for cesarean delivery (OR, 1.21 [CI, 1.01 to 1.46]), but this difference was not statistically significant in women at less than 41 completed weeks of gestation (OR, 1.73 [CI, 0.67 to 4.5]). Women who were expectantly managed were more likely to have meconium-stained amniotic fluid than those who were electively induced (OR, 2.04 [CI, 1.34 to 3.09]).

Limitations:

There were no recent RCTs of elective induction of labor at less than 41 weeks of gestation. The 2 studies conducted at less than 41 weeks of gestation were of poor quality and were not generalizable to current practice.

Conclusion:

RCTs suggest that elective induction of labor at 41 weeks of gestation and beyond is associated with a decreased risk for cesarean delivery and meconium-stained amniotic fluid. There are concerns about the translation of these findings into actual practice; thus, future studies should examine elective induction of labor in settings where most obstetric care is provided.

Induction of labor is increasing in the United States—from 9.5% of births in 1990 to 22.1% of births in 2004 (1, 2). Labor may be induced because of maternal (for example, diabetes mellitus, unstable cardiac disease, hypertensive disease of pregnancy) or fetal (for example, nonreassuring results on antenatal testing, intrauterine growth restriction) indications. Induction of labor without a medical indication is termed elective induction of labor and appears to be increasing even more rapidly than induction of labor as a whole (3–5).

Elective induction may be motivated by a variety of reasons. For example, pregnant women may wish to end their pregnancy because of physical discomfort; concern that rapidly progressing labor would preclude timely arrival at the hospital or epidural placement; scheduling issues; or ongoing concerns for maternal, fetal, or neonatal complications (1). Clinicians caring for pregnant women (such as obstetricians, family practice physicians, and midwives) may have similar nonmedical reasons for recommending elective induction of labor for their patients (4). They, too, may wish to end the ongoing risk for complications in the pregnancy, limit their patients' physical discomfort, or reduce the risks imposed by geographic barriers (6, 7). Clinicians may also have an incentive to use elective induction for their own economic benefit and scheduling preferences. Thus, it is imperative to characterize the potential maternal and neonatal outcomes associated with elective induction of labor.

Elective induction of labor necessarily reduces some risks of an ongoing pregnancy, such as development of preeclampsia, oligohydramnios, macrosomia, or intrauterine fetal demise at a later gestational age. Randomized, controlled trials have compared the rates of cesarean delivery between women with induction of labor and those with expectant management of pregnancy, and have generally concluded that the cesarean rate was unchanged or lower among the induced group (8, 9). However, the commonly held dogma regarding induction of labor is that it increases the risk for cesarean delivery (10), which in turn is associated with a host of maternal complications. In addition, a cesarean delivery in the current pregnancy increases both maternal and neonatal risks in future pregnancies (11, 12).

One critical aspect of the existing literature is the control group used for comparison with elective induction of labor. For a pregnant woman at a particular gestational age, the choices are to expectantly manage the pregnancy (no intervention) or to intervene with an induction of labor. Expectant management of the pregnancy allows the pregnancy to progress to a future gestational age. Thus, the woman undergoing expectant management may go into spontaneous labor, or she may require a medically indicated induction of labor at a future gestation because of developing preeclampsia, nonreassuring results on antenatal testing, or postterm pregnancy (8). One methodological problem with many studies of induction of labor, particularly observational studies, is that women in spontaneous labor are used as a control group. This is not a realistic comparison because women and their providers actually face the choice between induction of labor and expectant management, not spontaneous labor (13). Thus, in studies evaluating the risks and benefits of elective induction of labor, women undergoing elective induction of labor should be compared with women having expectant management of the pregnancy rather than women undergoing spontaneous labor.

The effect of elective induction of labor on the frequency of cesarean delivery is a critical uncertainty. An understanding of the effect of induction of labor on cesarean delivery would help clinicians and policymakers determine the benefits and harms, and thus define a reasonable role for elective induction of labor in current obstetric practice. In this review, we evaluated the published evidence on the maternal and neonatal risks of elective induction relative to expectant management of pregnancy. We also evaluated the evidence comparing elective induction of labor with spontaneous labor to demonstrate the reasons for the currently held opinion about the effect of elective induction of labor on cesarean delivery.

Methods

Data Sources and Searches

We searched MEDLINE (from 1966 to February 2009), Web of Science, CINAHL, and the Cochrane Central Register of Controlled Trials (up to March 2009) to identify all English-language studies on elective induction of labor in humans. We also manually reviewed the reference lists of included articles and bibliographies of systematic reviews to identify additional relevant articles. The ⁷, presents the details of our search strategy.

Study Selection

We sought studies that reported maternal and neonatal outcomes for women who had induction of labor without a specific indication during the term period of pregnancy, at or after 37 weeks and before 42 weeks of gestation; beyond 42 weeks is defined as a postterm pregnancy by the American College of Obstetricians and Gynecologists and is a medical indication for induction of labor. We included studies on elective induction of labor only if the article reported mode of delivery (that is, cesarean, spontaneous vaginal, or operative vaginal deliveries) or maternal or fetal and neonatal outcomes. We excluded articles that only compared different methods of induction of labor. We included duplicate articles of the same study only once in our analyses. We included only articles published from 1966 and beyond to represent modern obstetric practice.

Study Design

We included randomized, controlled trials (RCTs), cohort studies, and case–control studies. Because most RCTs compared elective induction of labor with expectant management and most of the observational studies compared elective induction of labor with spontaneous labor, we included all 3 study designs and both types of controls but analyzed them separately. The fundamental comparison for the study was between elective induction of labor and expectant management of pregnancy. Because the comparison between elective induction of labor and spontaneous labor is commonly reported in the observational literature, we present these findings primarily to demonstrate the current state of the existing literature as well as to explore the differential findings between the RCTs and observational studies.

Data Extraction and Quality Assessment

Two authors independently reviewed the title and abstract of all studies retrieved from our searches to assess whether the article met inclusion criteria. They then reviewed and extracted the following information from each included study: study period, location and setting of the study, method used to achieve labor induction, study design, mode of delivery, maternal and neonatal outcomes, and quality assessment variables. When the reviewers disagreed on the data abstracted, a third reviewer abstracted the data as well to resolve the differences. The resolution was agreed upon by all 3 reviewers.

Consistent with the Agency for Healthcare Research and Quality (AHRQ) draft “Grading the Strength of a Body of Evidence When Comparing Medical Interventions” (14), we developed specific criteria for evaluating the quality of the individual included studies and for assessing the applicability of these studies. Our quality assessments were based on the extent to which the included studies had a prospective design, compared women undergoing elective induction of labor with women being managed expectantly, treated key factors affecting cesarean delivery rates (such as maternal age, parity, body mass index, cervical stage, and gestational age) similarly in intervention and control patients, and were adequately powered to evaluate relatively rare outcomes of interest for both mothers and neonates. We summarized the results of our quality appraisal of the studies by using stacked bars, as has been done in previous systematic reviews (15–17). These assessments were also summarized as good, fair, or poor ratings for each individual study. We assessed the applicability of the individual studies by evaluating the population studied, place and time the study was conducted, and methods of induction used. Individual applicability was assessed as good, fair, or poor. To grade the overall strength of evidence, we considered the quality and applicability of the individual studies, the consistency of the results across the included studies, and volume of the literature. Each outcome examined was then assigned a grade of high, moderate, low, or insufficient to represent overall quality.

Data Synthesis and Analysis

We computed 2 summary effect sizes by using random-effects models for each outcome of interest reported by more than 4 studies: a summary odds ratio (OR) and a summary risk difference. We present the summary OR as the primary outcome metric in our figures and text, and we also provide the summary risk difference when applicable. The summary ORs were created such that a value greater than 1.0 means that expectant management of the pregnancy is associated with a higher risk for a particular outcome. We also conducted stratified (subgroup) analyses to evaluate the effect of the following variables on our outcomes of interest: year (1990 and earlier vs. after 1990), country (United States vs. a country other than the United States), gestational age (before or after 41 completed weeks of gestation), and setting (academic, community hospital, both, or multicenter). We defined study year as the year in which the study was started; if this was not reported, then we used the publication year for the study year. We assessed the statistical heterogeneity for all computed summary effects by calculating the Q statistic (designated Q statistic with a P value <0.05 was considered heterogeneous) and I² statistic (designated I² >50% was considered heterogeneous) (18–20).

We performed sensitivity analyses to evaluate the robustness of our results. We removed each study individually to evaluate that study's effect on the summary estimates. We assessed publication bias by visual inspection of funnel plots to evaluate the association between the sample size of a study and the likelihood of that study reporting a statistically significant outcome. We performed analyses by using Comprehensive Meta-Analysis software, version 2 (Biostat, Englewood, New Jersey).

Role of the Funding Source

This study was funded by AHRQ through its funding of the Stanford–University of California, San Francisco, Evidence-based Practice Center. Representatives from AHRQ participated in the conversations with the Technical Expert Panel and in review of the manuscript before its submission, but they did not influence the analyses, the preparation of the manuscript, or the decision to publish the manuscript.

Results

General Description of Search Results and Study Quality

Our search strategy yielded 6117 published articles. Of these, 36 studies compared maternal or neonatal outcomes in women who had elective induction of labor with outcomes in women who had expectant management or spontaneous labor (Figure 1).

Figure 1.

Study flow diagram.

General Description of the RCTs

In 9 RCTs (8, 9, 21–30), women who were expectantly managed were designated as the control group, and in 2 RCTs (31, 32), the control group consisted of women who had spontaneous labor (that is, women in the control group who were induced later were excluded from the analysis or analyzed as the induction group) (Table 1, and Appendix Tables 1, 2, and 3). The studies were conducted between 1975 and 2005. Seventy-three percent of these studies were conducted outside the United States (Canada, Europe, Turkey, and Japan). About half the studies were conducted in an academic setting and about one third in a community hospital or across multiple centers; 1 article did not report the setting.

Table 1.

Characteristics of Randomized, Controlled Trials of Elective Induction of Labor

Table 1.

Appendix Table 1.

Quality Characteristics of Randomized, Controlled Trials of Elective Induction of Labor

Appendix Table 1.

Appendix Table 2.

Maternal Outcomes in Randomized, Controlled Trials of Elective Induction of Labor

Appendix Table 2.

Appendix Table 3.

Neonatal Outcomes in Randomized, Controlled Trials of Elective Induction of Labor Versus Expectant Management

Appendix Table 3.

The overall quality of the RCTs was generally fair (Figure 2 and Table 1, and Appendix Tables 1, 2, and 3). Most of the studies were relatively small (<400 patients) or medium (400 to 1000 patients) in size, and few had calculated sample sizes to determine whether the study had adequate power to address the primary study question. There was considerable heterogeneity among the studies of elective induction of labor; this heterogeneity, similar to the quality rating, also led to poor applicability ratings with respect to maternal and neonatal outcomes in most studies. The most likely source of heterogeneity was the large time span over which the studies were conducted. Obstetric management has changed significantly over the past 30 years, as has the baseline cesarean delivery rate. Because several of the RCTs were older, their applicability was downgraded as well.

Figure 2.

Quality assessment of RCTs (top) and observational studies (bottom) of elective induction of labor.

Shown are the percentages of the included studies that did (black bars) and did not (gray bars) fulfill each of the a priori–determined quality criteria. RCT = randomized, controlled trial.

General Description of the Observational Studies

Elective induction of labor was assessed in 25 observational studies. Only 1 cohort study compared elective induction of labor with expectant management (33, 34); the remaining studies (35–60) compared elective induction with spontaneous labor (Table 2, and Appendix Tables 4, 5, and 6). These were conducted between 1962 and 2006. Slightly less than half of the studies were conducted outside the United States (Canada, Australia, India, Thailand, Belgium, Hungary, France, the Netherlands, and Saudi Arabia). Most observational studies were rated as poor quality largely because they used an inappropriate comparison group for the actual clinical decision-making situation (Figure 2 and Table 2, and Appendix Tables 4, 5, and 6). About half of the studies were relatively small (<400 patients) or medium (400 to 1000 patients) in size, and only a few had calculated sample sizes to determine whether adequate power existed to address the primary study question.

Table 2.

Characteristics of Observational Studies of Elective Induction of Labor

Table 2.

Appendix Table 4.

Quality Characteristics of Observational Studies of Elective Induction of Labor

Appendix Table 4.

Appendix Table 5.

Maternal Outcomes in Observational Studies of Elective Induction of Labor

Appendix Table 5.

Appendix Table 6.

Neonatal Outcomes in Observational Studies of Elective Induction of Labor Versus Spontaneous Labor

Appendix Table 6.

Maternal Outcomes

Cesarean Delivery

Of the 9 RCTs that compared cesarean delivery in women who had elective induction of labor (n = 3017) with that in women expectantly managed (n = 3121), the combined summary OR slightly favored elective induction of labor (Figure 3); of note, 1 study had no reported cesarean deliveries in either group (28) and thus was excluded from the summary ORs. Expectant management of pregnancy was associated with a 22% increase in cesarean delivery (OR, 1.22 [95% CI, 1.07 to 1.39]; P < 0.01) and an absolute risk difference of 1.9 percentage points (CI, 0.2 to 3.7 percentage points; P = 0.03); the I² was 0.00. Most of these studies included only women at or beyond 41 completed weeks of gestation (OR, 1.21 [CI, 1.01 to 1.46]; P = 0.04). Three trials reported a non–statistically significant difference in risk for cesarean delivery among women who were induced at less than 41 completed weeks of gestation (OR, 1.73 [CI, 0.67 to 4.5]; P = 0.26), but these trials were of poor quality (21, 22, 28). As part of a sensitivity analysis, we included the 2 trials that compared elective induction of labor with spontaneous labor in the overall analysis, with similar results (OR, 1.21 [CI, 1.03 to 1.44]; P = 0.02).

Figure 3.

Comparison of cesarean delivery and MSF reported by the randomized, controlled trials of elective induction of labor versus expectant management.

CS = cesarean section; Mgt = management; MSF = meconium-stained fluid; NICHHD = National Institute of Child Health and Human Development; RR = relative risk. Top. Odds ratios and 95% CIs for each of the 8 studies that reported CS rates and the summary (random-effects) odds ratio. Bottom. Odds ratios and 95% CIs for the 6 trials that presented data on meconium-stained fluid.

Three RCTs (9, 22, 26) reported cesarean delivery as an outcome specifically among nulliparous women. These studies included a total of 506 nulliparous women: 256 in the expectant management group and 250 in the elective induction of labor group. Risk for cesarean delivery did not significantly differ between the 2 groups (OR, 1.67 [CI, 0.81 to 3.46]; P = 0.17). Thus, insufficient information exists with which to draw any conclusions about the effect of elective induction specifically in nulliparous women. The same 3 RCTs (9, 22, 26) also reported cesarean delivery among 367 multiparous women. In all 3 studies, the rate of cesarean deliveries was low, with 1 study reporting no events among the women who were expectantly managed and only 1 event among the women who underwent induced labor (22). The other 2 studies had 3 and 2 cesarean deliveries, respectively, among the women who were electively managed and only 1 and 2, respectively, among those who were induced (9, 26). Thus, there was insufficient information with which to draw any conclusions about the effect of elective induction in multiparous women.

When we stratified the studies according to those conducted in or before 1990 and those conducted after 1990, there was no statistically significant difference in the odds of cesarean delivery for either group. When we stratified the analysis by country (Appendix Figure), we found that the odds of cesarean delivery were higher in women who were expectantly managed than in those with elective induction of labor in studies conducted outside the United States (OR, 1.21 [CI, 1.05 to 1.40]; P < 0.01; n = 5172) but were not statistically different in studies conducted in the United States (OR, 1.28 [CI, 0.65 to 2.49]; P = 0.47; n = 966), which appeared to be heterogeneous (I² = 68).

Appendix Figure.

Comparison of cesarean delivery reported by the randomized, controlled trials of elective induction of labor versus expectant management, stratified by study location.

CS = cesarean section; Mgt = management; NICHHD = National Institute of Child Health and Human Development; RR = relative risk.

The observational studies reported a consistently lower risk for cesarean delivery among women who underwent spontaneous labor (5%) than women who had elective induction of labor (7%), with a statistically significant decrease when combined (OR, 0.65 [CI, 0.52 to 0.81]; P < 0.01).

Considering the quality and quantity of the body of evidence and its applicability to obstetric care in the United States today, the evidence for the relationship between elective induction of labor at 41 0/7 weeks of gestation and cesarean delivery was rated as moderate. However, with respect to elective induction of labor before 41 0/7 weeks of gestation, the overall evidence was considered insufficient.

Operative Vaginal Delivery

An operative vaginal delivery refers to a forceps- or vacuum-assisted vaginal delivery. Most of the 6 RCTs that examined the effect of elective induction of labor on operative vaginal delivery were small- to medium-sized studies (only 1 study had 1700 women in each group) (8, 21, 22, 25, 26, 28). The summary odds of operative vaginal delivery were not statistically different between women who were electively induced and women who were expectantly managed (OR, 0.91 [CI, 0.79 to 1.04]; P = 0.18). Three RCTs reported no difference in the risk for operative vaginal delivery among women who were induced at less than 41 0/7 weeks' gestational age (OR, 0.71 [CI, 0.41 to 1.21]; P = 0.21), but all of these trials were of poor quality (21, 22, 28). For the 7 observational studies, the risk for operative vaginal delivery did not significantly differ between women in spontaneous labor and those having elective labor induction (OR, 0.91 [CI, 0.8 to 1.05]; P = 0.18) (35, 37, 38, 40, 46, 51, 52, 54). Given the consistency of the findings in both RCTs and observational studies for a lack of difference in the risk for operative vaginal delivery, the overall evidence regarding the relationship between elective induction of labor and operative vaginal delivery was determined to be moderate.

Maternal Infections

Six studies (3 RCTs [8, 26, 27] and 3 observational studies [37, 46, 50]) reported the presence or absence of maternal infection; however, none provided detailed quantitative data, such as risk ratios or risk differences. Four studies (2 RCTs and 2 observational studies) provided some evidence that elective induction was not associated with an increased risk for chorioamnionitis, and 2 observational studies provided some evidence that elective induction was not associated with an increased risk for endomyometritis. Given the consistency in these findings but the modest amount of available data, the overall strength of evidence for maternal infections was rated as low.

Maternal Blood Loss and Hemorrhage

Five studies (1 RCT [25] and 4 observational studies [35, 37, 46, 50]) found no link between elective induction and postpartum hemorrhage. However, these studies probably lacked adequate statistical power to detect a difference. Given the limited number of studies, the evidence on the effect of elective induction of labor on maternal hemorrhage was considered insufficient.

Other Maternal Outcomes

Two studies (1 RCT [25] and 1 observational study [46]) reported results on serious perineal lacerations and did not observe an association between elective induction and third- or fourth-degree perineal lacerations. No studies addressed the risk for hysterectomy, length of labor, evidence of injury to internal organs, or wound complications after elective induction of labor. Given the limited information available on these other outcomes, the evidence on the effect of elective induction of labor was considered insufficient.

Neonatal Outcomes

Meconium-Stained Amniotic Fluid

Six randomized, controlled studies enrolling 5478 women examined whether the presence of meconium-stained amniotic fluid (a sign of postmaturity and intrauterine fetal stress) was associated with elective induction of labor (Figure 3) (8, 9, 21, 23, 25, 27). The overall rate (±SE) of meconium-stained amniotic fluid in the expectant management group was 29% ± 0.04% compared with 17% ± 0.06% in the elective induction of labor group. When combined, these studies showed that the presence of meconium-stained amniotic fluid was more likely in the expectant management group than in the elective induction of labor group (OR, 2.04 [CI, 1.34 to 3.09]; P < 0.01), but the studies were heterogeneous (I² = 83). Given the consistency of the findings and the quality of the individual studies, which ranged from poor to good, the overall evidence regarding the presence of meconium was rated as moderate.

The Meconium Aspiration Syndrome

Five randomized, controlled trials provided somewhat conflicting results on the effect of elective induction on the meconium aspiration syndrome (8, 9, 23, 25, 27). Although 2 of the studies found higher rates of meconium aspiration in the setting of expectant management, these differences were not statistically significant; the other 3 studies found no difference. Overall, the risk for the meconium aspiration syndrome in neonates did not differ between the 2 groups of women (OR, 1.39 [CI, 0.71 to 2.72]; P = 0.35). Thus, more data are needed to further evaluate the presence and strength of this association, and the overall evidence regarding meconium aspiration was considered low.

Apgar Score Less Than 7 at 5 Minutes

Thirteen studies (4 RCTs [8, 9, 25, 26] and 9 observational studies [36–38, 40, 46, 51, 52, 54, 60]) provided evidence that the rate of a 5-minute Apgar score less than 7 was no different between women undergoing elective induction of labor and those having expectant management or spontaneous labor. The summary OR from the RCTs was 1.18 (CI, 0.67 to 2.06; P = 0.57). Given the relatively wide CI, that this outcome is relatively uncommon and lacks adequate power, and the individual quality ratings of the studies, the overall evidence regarding this outcome was rated as low.

Admissions to Neonatal Intensive Care Unit

Three RCTs reported on admissions to the neonatal intensive care unit (23, 25, 26). There was no difference in the odds of admissions between the groups receiving elective induction of labor or expectant management (OR, 1.24 [CI, 0.73 to 2.09]; P = 0.43). Given the relatively wide CI and the individual quality ratings of the studies, the overall evidence for this outcome was rated as low.

Other Neonatal Outcomes

For several key neonatal outcomes, a few good-quality RCTs reported evidence that was rated as low quality (typically because of the small number of studies). These include the finding of no difference among women with elective induction of labor and those with expectant management for the neonatal risk for transient tachypnea, suspected sepsis, seizures, hypoglycemia, jaundice, polycythemia, or low birthweight. Evidence for many key neonatal outcomes of interest was insufficient to allow any conclusions to be drawn, including rates of neonatal death, neonatal acidemia, fetal distress, the fetal respiratory distress syndrome, and initiation of successful breastfeeding.

Discussion

The key finding of this systematic review is that the likelihood of cesarean delivery appears to be equivalent or lower in women who were electively induced compared with those who were expectantly managed. This finding contrasts with the widely held opinion about elective induction of labor, but it is consistent with existing meta-analyses comparing studies of induction of labor with those of expectant management of pregnancy (61, 62). In a recent Cochrane Collaboration review (62) that examined induction of labor in women at less than 41 weeks' gestation, the rate of cesarean delivery was significantly reduced in the elective induction group (OR, 0.58 [CI, 0.34 to 0.99]). This statistically significant finding was largely due to a French-language study, published in 1982, that was conducted in France and included more than 700 women (63). The authors found that among women who were induced, 19 of 481 (4%) had cesarean delivery versus 16 of 235 (7%) women with expectant management. Although the reduction in cesarean delivery is promising, it is difficult to generalize these study findings to current obstetric practice because this study was conducted more than 25 years ago and outside the United States, during a time when the cesarean delivery rate was 3 to 5 times lower than it is today.

The finding of a reduced cesarean rate among women who are electively induced contradicts the commonly held opinion that induction of labor actually increases the risk for cesarean delivery. This belief is supported by the observational literature, which has compared induction of labor with spontaneous labor, and generally has found a higher rate of cesarean delivery in women who are induced. However, because the actual choice faced by clinicians and their patients is induction of labor or expectant management of the pregnancy, the comparison of induction of labor with spontaneous labor as a methodological approach to evaluating elective induction of labor does not produce results that are clinically relevant or that can be used to counsel women prospectively (13). When considering why the cesarean delivery rate may be lower in women induced than in those expectantly managed, we look at the findings of indication for cesarean delivery from the study by Hannah and colleagues (8). Although they found no difference in the rates of cesarean delivery for the combined indications of labor dystocia or failed induction of labor, there was a specific difference in cesarean deliveries for nonreassuring fetal heart rates (5.7% in the induction of labor group vs. 8.3% in the expectant management group). Fundamentally, there are only 2 overall reasons for a cesarean delivery in labor: cephalopelvic disproportion (commonly diagnosed as failure to progress in labor) or fetal intolerance of labor, which may be due to reduced utero–placental blood flow. Because fetuses continue to grow and placentas continue to age throughout term pregnancies, it is not surprising that these indications might increase with gestational age. The competing risk is failed induction of labor, which probably varies from institution to institution.

A moderate amount of evidence from the current report and previous meta-analyses shows that induction of labor at 41 completed weeks of gestation and beyond leads to a lower rate of cesarean delivery. Our analysis specifically included only studies that randomly assigned patients before 42 weeks of gestation, in accordance with the current definition of elective induction of labor by the American College of Obstetricians and Gynecologists. However, far fewer RCTs have addressed this question at earlier gestational ages. Moreover, translation of these findings to the population at large in various practice settings has not been well studied. How elective induction of labor may be used in nonstudy settings requires careful consideration by policymakers, clinicians, and patients alike to avoid an expensive intervention that actually may increase cesarean delivery and associated morbidity in current and future pregnancies.

Beyond cesarean delivery, moderate evidence indicates that the incidence of meconium-stained amniotic fluid is decreased by elective induction of labor; however, examination of most other outcomes demonstrates no statistically significant differences and provides low or insufficient evidence. Thus, the safety of elective induction labor requires further investigation. Future studies should be designed to include the appropriate comparison group—namely, women whose pregnancy is expectantly managed. In addition, they should report a wide variety of maternal outcomes, including estimated blood loss; incidence of postpartum hemorrhage, chorioamnionitis, endomyometritis, and perineal lacerations; epidural use; and length of hospital stay, as well as uncommon but severe conditions, such as pulmonary embolus, amniotic fluid embolus, hysterectomy, and mortality. In addition to the more traditional clinical outcomes, economic and quality-of-life measures such as patient preferences or utilities, should also be considered in future studies of elective induction of labor.

Our systematic review had several limitations. First, the inclusion of only English-language studies resulted in exclusion of one of the largest RCTs on induction of labor, which was published in French (63). However, given time and budgetary limitations, it was not feasible to translate non–English-language reports. Having reviewed other related meta-analyses and foreign-language studies with English abstracts, we believe this is the only pertinent study not reported in English that we did not identify in our search. The most important limitation of our review was the relatively small number of well-designed, adequately powered studies reporting on the maternal and fetal outcomes of interest; there were only 9 prospective RCTs: 2 (22%) of good quality, 4 (44%) of fair quality, and 3 (33%) of poor quality. For most outcomes, there were no more than 5 studies. Synthesis of the literature with such few studies is challenging because a single study may affect the outcomes and introduce heterogeneity; in addition, there were too few studies to allow us to detect publication bias. Furthermore, because specific methods of induction were not the focus of these studies, which usually included multiple management styles, it is difficult to provide guidance on optimal methods of induction. However, given the improvements in induction of labor methods with widespread use of cervical ripening agents, one can presume that the rates of failed induction of labor should only be lower in current practice.

In this systematic review of elective induction of labor, we found that overall elective induction of labor as compared with expectant management of the pregnancy was associated with an approximately 20% reduction in the rate of cesarean delivery and a 50% reduction in the presence of meconium in the amniotic fluid. These results require further examination in large randomized trials before routine adoption into clinical practice in women before 41 weeks of gestation. Such trials should endeavor to stratify by maternal characteristics, such as parity and cervical status at time of randomization, as well as method of induction of labor. Furthermore, well-designed observational studies could also examine the outcomes between women undergoing elective induction of labor as compared with those expectantly managed in a variety of nonacademic settings to look at the generalizability of these findings. Finally, because of the heterogeneity in the management of labor induction, which varies widely between providers and institutions, careful examination of the effect of such policies in many settings should be explored before elective induction of labor is routinely adopted as a potential policy to prevent complications of term pregnancies.

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Appendix: Search Strategy

Original Search

MEDLINE Search 1

“Puerperal Disorders”[MeSH] OR “Pregnancy, Prolonged”[MeSH] OR “Pregnancy Complications”[MeSH] OR “Morbidity”[MeSH] OR “Infant, Newborn”[MeSH] OR “Fetal Diseases”[MeSH] OR “Puerperal Infection”[MeSH] OR “Pregnancy Outcome”[MeSH] OR “Cesarean Section”[MeSH] OR “Obstetric Labor Complications”[MeSH] AND “Labor, Induced”[MeSH]

Limits: English, Humans

# of citations as of 26 February 2009: 2735

MEDLINE Search 2

#1 “Labor, Induced”[mesh] OR “induction of labor” OR “labor induction” OR (labor[ti] OR labour[ti] AND induc*[ti])

#2 trial OR study OR studies OR follow* OR “long-term” OR outcome* OR risk OR mortality OR fatal* OR disabilit* OR (“Treatment Outcome”[MeSH] OR “Pregnancy Outcome”[MeSH] OR “Controlled Clinical Trials”[MeSH] OR “Epidemiologic Factors”[MeSH] OR “Cohort Studies”[MeSH] OR “Risk”[MeSH] OR “Retrospective Studies”[MeSH] OR “Prognosis”[MeSH] OR “Mortality”[MeSH] OR “Follow-Up Studies”[MeSH] OR “mortality”[Subheading] OR “Controlled Clinical Trial”[Publication Type])

#3 #1 and #2, limits: English, Humans

# of citations as of 26 February 2009 (total): 3408

# of citations as of 26 February 2009 (unique): 1189

MEDLINE Search 3

#1 “Labor, Induced”[mesh] OR “induction of labor” OR “labor induction” OR (labor[ti] OR labour[ti] AND induc*[ti]) AND elective

# of citations as of 26 February 2009 (total): 295

# of citations as of 26 February 2009 (unique): 15

Total number of unique citations from original MEDLINE search above as of 26 February 2009: 3939

Updated and Expanded Search

MEDLINE Search 1: Labor AND Induction AND Pregnancy Disorders

((“Labor, Induced”[MeSH] OR “induction of labor” OR “induced labor” OR “labor induction” OR ((labor[tw] OR labour[tw]) AND (initiate*[tw] OR initiating[tw] OR induc*[tw]))) AND (“Puerperal Disorders”[MeSH] OR “Pregnancy, Prolonged”[MeSH] OR “Pregnancy Complications”[MeSH] OR “Morbidity”[MeSH] OR “Fetal Diseases”[MeSH] OR “Puerperal Infection” [MeSH] OR “Pregnancy Outcome”[MeSH] OR “Cesarean Section”[MeSH] OR “Obstetric Labor Complications”[MeSH] OR “Infant, Newborn”[MeSH]) AND ((“2007”[EDat] : “3000” [EDat]) AND (Humans[Mesh]) AND (English[lang]))) NOT (Editorial[ptyp] OR Letter[ptyp] OR Case Reports[ptyp])

# of citations from 2007 to 10 March 2009: 387

MEDLINE Search 2: Labor AND Induction

((“Labor, Induced”[MeSH] OR “induction of labor” OR “induced labor” OR “labor induction” OR ((labor[tw] OR labour[tw]) AND (initiate*[tw] OR initiating[tw] OR induc*[tw])) AND ((“2007”[EDat] : “3000”[EDat]) AND (English[lang]))) NOT (“Labor, Induced”[MeSH] OR “induction of labor” OR “induced labor” OR “labor induction” OR ((labor[tw] OR labour[tw]) AND (initiate*[tw] OR initiating[tw] OR induc*[tw])) AND ((“2007”[EDat] : “3000”[EDat]) AND (English[lang]) AND (Editorial[ptyp] OR Letter[ptyp] OR Case Reports[ptyp])))) NOT (animals[mh] NOT human[mh])

# of citations from 2007 to 10 March 2009: 711

MEDLINE Search 3: Elective AND Labor AND Induction

electiv* AND (((“Labor, Induced”[MeSH] OR “induction of labor” OR “induced labor” OR “labor induction” OR ((labor[tw] OR labour[tw]) AND (initiate*[tw] OR initiating[tw] OR induc*[tw])) AND ((“2007”[EDat] : “3000”[EDat]) AND (English[lang]))) NOT (“Labor, Induced”[MeSH] OR “induction of labor” OR “induced labor” OR “labor induction” OR ((labor[tw] OR labour[tw]) AND (initiate*[tw] OR initiating[tw] OR induc*[tw])) AND ((“2007”[EDat] : “3000”[EDat]) AND (English[lang]) AND (Editorial[ptyp] OR Letter[ptyp] OR Case Reports[ptyp])))) NOT (animals[mh] NOT human[mh]))

# of citations from 2007 to 10 March 2009: 59

Total number of unique citations from expanded MEDLINE search above as of 10 March 2009: 711

Web of Science

Search 1: Elective AND Labor AND Induction (no date restriction)

Topic = (electiv*) AND Topic = ((labor OR labour)) AND Topic = ((initiate* OR initiating OR induc*)) Refined by: [excluding] Document Type = (LETTER OR PROCEEDINGS PAPER OR EDITORIAL MATERIAL OR MEETING ABSTRACT) AND Languages = (ENGLISH) Databases = SCI-EXPANDED, SSCI, A&HCI Timespan = All Years

# of citation as of 10 March 2009: 393

Search 2: Labor AND Induction AND StudyTypes/Comparison Terms

Topic = ((cohort OR risk OR retrospectiv* OR prospectiv* OR prognos* OR mortality OR compar* OR trial OR study OR studies OR follow* OR “long-term” OR outcome* OR risk OR mortality OR fatal* OR disabilit*)) AND Title = ((labor OR labour)) AND Topic = ((initiate* OR initiating OR induc*)) Refined by: [excluding] Document Type = (LETTER OR PROCEEDINGS PAPER OR NOTE OR MEETING ABSTRACT OR EDITORIAL MATERIAL) AND Languages = (ENGLISH) Timespan = All Years. Databases = SCI-EXPANDED.

# of citations as of 10 March 2009: 973

Total number of unique citations from Web of Science search as of 10 March 2009: 1231

CINAHL (No Date Restrictions)

(MH “Labor, Induced+”) or (((labor OR labour) AND (initiate* OR initiating OR induc*))) NOT Publication Type: Abstract, Book, Book Chapter, Book Review, Case Study, Commentary, Conference, Editorial, Letter NOT MEDLINE. English only.

Total number of citations from CINAHL as of 10 March 2009: 472

Cochrane Central Register of Controlled Trials (No Date Restrictions)

(((labor or labour) and (initiate* or initiating or induc*)).mp. [mp = title, original title, abstract, mesh headings, heading words, keyword] NOT MEDLINE, Case Report, Letter

Total number of citations from Cochrane Central Register of Controlled Trials as of 10 March 2009: 667

Total number of citations from updated and expanded search: 2917

Total number of unique citations from updated and expanded search as of 10 March 2009: 2178