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Caughey AB, Sundaram V, Kaimal AJ, et al. Maternal and Neonatal Outcomes of Elective Induction of Labor. Rockville (MD): Agency for Healthcare Research and Quality (US); 2009 Mar. (Evidence Reports/Technology Assessments, No. 176.)

  • This publication is provided for historical reference only and the information may be out of date.

This publication is provided for historical reference only and the information may be out of date.

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Maternal and Neonatal Outcomes of Elective Induction of Labor.

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4Discussion

Overall, we found consistent evidence from the randomized controlled trials included in our analysis that elective induction of labor led to either no difference or a slightly decreased rate of cesarean delivery at 41 weeks of gestation; there was minimal evidence to suggest that elective induction of labor at this gestational age would lead to an increase in the rate of cesarean delivery. Further, the studies did not generally find an increase in either maternal or neonatal complications in the setting of elective induction of labor at 41 weeks of gestation and potentially a decrease in the presence of meconium. Unfortunately, there was insufficient evidence to examine outcomes prior to 41 weeks of gestation. In our decision and cost-effectiveness analyses, elective induction of labor was associated with improved maternal and neonatal outcomes and to be a cost-effective alternative to expectant management of pregnancy.

The quantity of evidence from RCTs for Key Questions 1 and 2 was somewhat limited. However, there was an overall slight increase in the odds of cesarean delivery with expectant management (OR 1.22; 95 percent CI 1.07–1.39) compared to elective induction of labor. Interestingly, in a stratified analysis of U.S. and non-U.S. studies, there was little difference between elective induction of labor and expectant management of pregnancy in studies conducted in the U.S.; however, there was a consistent increase in cesarean delivery with expectant management of pregnancy in the studies conducted outside of the U.S. (Figure 2.8). The majority of prospective studies examined women at 41 weeks of gestation. When we stratified to studies that examined women prior to 41 weeks of gestation, we did not find a statistically significant difference in the effect on cesarean delivery. The quality of the evidence examining the Key Questions ranged from insufficient to moderate, with much more of the former than the latter. Further, the applicability to Key Questions 1 and 2, in particular, was poor to fair for many of them except for the more recently conducted randomized trials from the U.S.

These findings are generally consistent with existing meta-analyses which include studies of induction of labor as compared to expectant management of pregnancy.90, 91 In the study by Sanchez-Ramos et al., a meta-analysis of postterm pregnancy that included women at 41 and 42 weeks gestation, there was a reduction in the rate of cesarean delivery in women who were induced (OR 0.88; 95 percent CI 0.78 – 0.99) as compared to those women who underwent expectant management.90 In a recent Cochrane review which stratified groups by gestational age, there was a non-significant decrease in the rate of cesarean delivery in women who underwent induction of labor (OR 0.92; 95 percent CI 0.76 – 1.12).91 Interestingly, in the group of studies of women less than 41 weeks gestation, there was a reduction in the rate of cesarean delivery in the elective induction group (OR 0.58; 95 percent CI 0.34–0.99). This statistically significant finding was, in large part, due to a study reported in French (which the current report excluded due to the inclusion criteria of only studies published in English language) of over 700 women published in 1982. Using a 2:1 randomization scheme in favor of elective induction of labor, this study randomized women at 37–39 weeks of gestation. In women who were induced, 19/481 (4 percent) women were delivered via cesarean versus 16/235 (7 percent) of the women with expectant management. While the reduction in cesarean delivery is promising, it is difficult to generalize these study findings to current obstetric practice since this was a study conducted more than 25 years ago in a different clinical setting as compared to the current practice environment in the United States; particularly, it was during a time period when the cesarean delivery rate during labor was 3 to 5 times lower than it is today.

The evidence from cohort and case-control studies is mixed, though generally, these observational studies either support that elective induction of labor is associated with an increase in cesarean delivery or show no difference in risk. Because of the heterogeneity in these studies, we were unable to perform a formal synthesis of the data. However, the majority of these studies utilized a control group consisting of women in spontaneous labor as opposed to expectant management of pregnancy. In the single observational study which utilizes an expectantly managed group of women as the control group,31 the authors reported an increase in cesarean delivery in those managed expectantly. The conflicts in study results between these studies as well as the differences in study design (observational studies and RCTs) will be discussed in detail below.

Importance of Methodologic Differences Between Interventional and Observational Studies

The effect of the study design differences on the reported cesarean delivery rate as well as other perinatal outcomes observed between RCTs versus cohort and case-control studies must be addressed. A similar effect was observed in a meta-analysis of induction of labor in the setting of presumed macrosomia by Sanchez-Ramos in 2002 that reported a larger increase in the effect of induction of labor on cesarean delivery seen in the observational studies.144 As previously discussed, this difference may be due to both measurable and residual confounding bias, even after attempts are made to control for confounding using multivariable regression techniques. However, it is likely that the study designs of traditional cohort and case-control studies which compare women undergoing induction of labor to those with spontaneous labor simply do not provide useful information on either mode of delivery or perinatal outcomes. Unfortunately, this flawed information may be used to facilitate informed clinical decision making or establish practice guidelines and policies. Such a comparison of induction versus spontaneous labor is impossible for clinicians and their patients to make in the clinical setting as they are faced with the decision of either employing induction of labor or continuing expectant management of the pregnancy, not spontaneous labor. Again, expectant management leads to either spontaneous labor or induction of labor at a later gestational age.

This is not to suggest that the results from randomized trials are infallible. Interestingly, three of the reports we excluded from our meta-analysis of induction of labor versus expectant management of the pregnancy were RCTs. These studies randomized women to induction of labor versus expectant management of pregnancy.63, 66, 145 However, in the analysis of the data, the authors either excluded those patients who ended up needing to be induced from the expectant management arm63, 66 or incorrectly allocated those women with inductions into the induction arm.145 It is particularly problematic to combine women who were induced for either abnormal antenatal testing or postterm pregnancy with those undergoing elective labor induction as each of the above factors are known to be associated with increased risk of cesarean delivery. Analyzing the data in this fashion would distort the appropriate study design since one goal of elective induction of labor is to avoid an indicated induction of labor.

As an example, we present the specifics from one of these studies.66 At the 38th week, all patients were admitted into the trial and examined. After exclusions, there were 230 women with a normal obstetrical history enrolled - 106 in the planned induction (treatment) group and 124 in the expectant management (control) group. The authors excluded 32 women from the control group secondary to either an obstetric complication necessitating induction or failure to begin spontaneous labor prior to 42 weeks gestation such that a postterm induction of labor was required, leaving 92 women who entered labor spontaneously. Thus, of 124 women randomized to the control group, 32 (26 percent) ultimately required induction of labor for either the development of an obstetric abnormality (e.g. preeclampsia, oligohydramnios, abnormal antenatal testing) or postterm pregnancy.

While RCTs are the best way to address the study questions, observational studies can be utilized to examine these question as well. If women who are induced at one gestational age are compared to all women who progress beyond that specific gestational age who then experience either spontaneous labor or induced labor, then this design scheme creates similar comparison groups as those seen in a well designed RCT. Among the observational studies we identified, one study attempted to utilize this designation of comparison groups.31; this was a retrospective cohort study conducted in Hungary based on the care of a single group of obstetricians was published in 1986. The authors compared women who were electively induced to those who had not been electively induced and either went into spontaneous labor or were induced postterm at 42 weeks of gestation. They reported a cesarean delivery rate of 1 percent in the women who were electively induced, as oppose to a 6.7 percent risk of cesarean in those who were expectantly managed. Additionally, they reported better neonatal outcomes in the induction group, with lower rates of Apgar score of 7 or less (3.7 percent versus 17.6 percent), neonatal morbidity (3.4 percent versus 7.0 percent), and neonatal mortality (0.5 percent versus 1.7 percent). However, while this study is supportive of elective induction of labor, it is likely profoundly confounded by both cervical status (women were only induced if they had a favorable cervix) and parity.

Recently, an observational study utilizing a study design which compared induction of labor to expectant management of pregnancy demonstrated how such design issues affect the findings by gestational age.3 In this study, when the authors used the traditional comparison between induction of labor and spontaneous labor, induction of labor was associated with an increased risk of cesarean delivery. However, when they compared women who were induced at a specific gestational age to those undergoing expectant management of pregnancy (i.e., women who either went into spontaneous labor or were induced beyond that particular gestational age), the authors reported a decreased risk of cesarean delivery in the women who were induced.

Preventive Induction of Labor

Another series of studies that have utilized the retrospective cohort or case-control study designs which have captured the essence of comparing elective induction of labor versus expectant management are the studies by Nicholson et al.146, 147 These studies examine the use of preventive induction of labor known as active management of risk in pregnancy at term (AMOR-IPAT). This clinical management strategy is designed to identify women who are at an increased risk for a cesarean delivery during labor from either fetal intolerance of labor due to placental insufficiency or failure to progress in labor from cephalo-pelvic disproportion.146 The authors have identified a series of risk factors for each of the indications for cesarean delivery (Table 4.1) and assigned these risks a number of days which are used to determine the upper limit of the optimal time of delivery (UL-OTD).148 The associated days from these risk factors are summed and the total number is subtracted from 41 weeks of gestation. The patient is then induced at the resultant gestational age, but never prior to 38 weeks of gestation.

Table 4.1. Upper Limit of Optimal Time of Delivery Calculation Table.

Table 4.1

Upper Limit of Optimal Time of Delivery Calculation Table.

There are two studies assessing the effects of this strategy in two different populations using a retrospective, observational study design. In the first study, a retrospective case-control study of AMOR-IPAT, 100 women who were exposed to this preventive induction strategy had labor induction 63 percent of the time and a 4 percent cesarean delivery rate as compared to 300 women receiving standard care which led to a lower rate of labor induction (26 percent) but a 17 percent cesarean delivery rate (P=0.01).146 In the second study, a retrospective cohort study, the 794 women who were exposed to AMOR-IPAT, had a higher induction rate (31 percent versus 20 percent respectively, P<0.001) and lower cesarean delivery rate (5 percent versus 12 percent respectively, P<0.001) as compared to the 1,075 women in the standard care group.147 Further, each of these studies demonstrated an improvement in a number of perinatal outcomes in the preventive induction of labor groups. These included thick meconium, NICU admissions, macrosomia, postpartum hemorrhage, and third- or fourth-degree perineal lacerations, without an increase in other maternal or neonatal complications.

There are obvious limitations to each of these studies. It is difficult to study process of care in a retrospective study. There are numerous potential confounding factors that may not be recognized or measured, thus resulting in residual confounding even after many of these differences between the populations have been accounted for by statistical techniques. For example, providers caring for the women using the preventive strategy may have been more motivated to achieve vaginal birth, leading to a higher level of patience in managing their patients' respective labor and delivery. Some of the potential confounding factors may be addressed by utilizing a prospective randomized controlled study design. In a pilot study which randomized women to AMOR-IPAT and standard care, the cesarean delivery rate was 10 percent in women managed with the AMOR-IPAT and 15 percent in women managed with standard care.149 While this difference was not statistically different, it does represent a 33 percent reduction in cesarean delivery in a modern obstetric setting.

Predictors of Mode of Delivery in the Setting of Induction

Our initial goal was to examine a variety of predictors of cesarean delivery in the setting of elective induction of labor. However, given the limitations of the included studies we were unable to perform the planned analysis. Thus we expanded our inclusion criteria to studies of women with any induction of labor as secondary levels of evidence, though we strictly excluded studies in which women were all postterm. Despite expanding our search beyond elective induction of labor, we still had difficulty in identifying a wide body of literature on this topic. As noted in Figure 2.23, the majority of items we identified as potential predictors were not examined in most of the studies we included in this review.

Of the factors characterized as predictors of success in the setting of induction of labor, two were clearly supported by evidence. Multiparity was associated with a lower cesarean delivery rate after induction of labor, thus a higher success rate using the measure of any vaginal delivery. Similarly, a “favorable” cervix examination, as measured by Bishop score or cervical length and dilation, was also associated with a lower rate of cesarean delivery. These findings are not surprising. Certainly, multiparity and cervical status are commonly cited throughout obstetrical textbooks and review papers as important factors in predicting success in the setting of a trial of labor.150 However, it is important to consider how these two factors are regarded in the setting of determining who may benefit from induction of labor. Since a cesarean delivery resulting from an induction of labor is considered a failure and is also associated with many of the maternal complications of labor and delivery, one may minimize this risk by electively inducing only multiparous women or those who have a favorable cervical status. However, the anticipated risk reduction may not be realized for the reasons we discuss below.

The Bishop score was initially used to predict who would go into labor within the following week and only later modified to predict labor induction success.151 Given that one of the benefits from elective induction of labor is to deliver the baby before any complications develop, women with a favorable Bishop score would then be the least likely to benefit from such an intervention. One may consider the question of induction of labor in terms of number needed to treat (NNT) and number needed to harm (NNH), stratified by Bishop score. When considering the number needed to harm, if we acknowledge the prevailing view that induction of labor increases the risk of cesarean delivery, then it follows that the number needed to harm is higher for women with a favorable Bishop score. Thus, inducing these women seems more reasonable. However, given that the findings from this review as well as the recent Cochrane review91 actually demonstrate a decrease in cesarean delivery in women who are electively induced, the opposite effect holds. The number needed to treat to achieve a benefit of lower cesarean delivery or lower neonatal complications would actually be lower in women who have an unfavorable cervix. This reduction stems from the lower baseline cesarean delivery rate in women with a favorable cervix, and also because the principle mechanism through which an elective induction of labor offers benefit is prevention of subsequent complications. Women with a favorable Bishop score are those destined for spontaneous labor shortly, and they have less preventive benefit potential than those with an unfavorable cervix.

Similarly for multiparous women, conventional wisdom in obstetrics has held that these women are less likely to be harmed from an elective induction. However, since the benefit of elective induction of labor is to those women who will not go into labor in the near future, these women are actually less likely to benefit from an elective induction. Interestingly, in a recent retrospective cohort study which used a study design to capture the control group of expectant management, a stratified analysis by parity was conducted.3 In nulliparous women, a decrease in cesarean delivery associated with induction of labor was illustrated. However, in multiparous women, there was either no difference or an increase in rate of cesarean delivery in women induced at 40 weeks of gestation as compared to those undergoing expectant management. Thus, the existing evidence would not support the conventional wisdom that multiparous women benefit most from elective induction of labor. We make recommendations on how studies might address the issues of parity and cervical status in the following section on future research.

Other predictors of a successful induction of labor were reported by few studies that were heterogeneous, and therefore no synthesis was performed. Several potential predictors are notable, including gestational age at delivery, maternal age at delivery, obesity, and amniotic fluid index, and are discussed below.

Gestational Age at Delivery

In the four cohort studies that examined gestational age at delivery, a greater gestational age at delivery was associated with a higher failure rate. Biologic plausibility exists for this observation. Once labor is achieved, there are fundamentally two reasons for a cesarean delivery: Fetal intolerance of labor and cephalo-pelvic disproportion. Since the placental function in term pregnancies appears to undergo functional decline with increasing gestation, fetal intolerance of labor would then likely increase with gestational age. Similarly, the fetus continues to grow as gestation progresses, subsequently increasing the probability of cephalo-pelvic disproportion with gestational age. These findings have been demonstrated in term pregnancies in a cohort of laboring women.130

Maternal Age

We found two retrospective cohort studies of maternal age and success of labor induction in our search of the published literature. The authors of the larger study reported an increase in cesarean delivery in women over the age of 35 as compared to women younger than 35. This finding mirrors the prevailing tenet in the overall literature regarding maternal age and the risk of cesarean delivery. This may reflect true biological differences seen between younger and older women. For instance, older women are more likely to experience complications of pregnancy such as preeclampsia and gestational diabetes. In addition, it appears that they also have longer labors. In a recent study in which maternal age was characterized by five-year subgroups, increasing maternal age was associated with longer length of labor.120 Perhaps, older women have a dysfunctional myometrium leading to higher likelihood of failure in labor. Alternatively, it may be that older women are being held to labor standards based on younger women and thus prematurely diagnosed with active phase arrest.

Obesity

There are several studies which have examined the association of body-mass index (BMI) and cesarean delivery in the setting of induction of labor. When maternal BMI was dichotomized at 30kg/m2, the authors found that those women with a higher BMI had higher rates of cesarean delivery in the setting of induction of labor. Again, this finding is consistent with the overall literature examining BMI and mode of delivery. The presumed biologic plausibility behind such an association includes higher rates of pregnancy-related complications such as preeclampsia, gestational diabetes mellitus, and increased birthweight. These factors are associated with maternal obesity leading to higher rates of cephalo-pelvic disproportion and cesarean deliveries.12, 118, 152

Diagnostic Bias with Identifying Predictors of a Successful Induction of Labor

One challenge not previously discussed when assessing predictors of a successful induction of labor is diagnostic bias. Clinicians are not blinded to their patient's clinical history and physical characteristics. Based on their clinical intuition, they may believe that an individual patient is more or less likely to achieve vaginal delivery. A multipara with a favorable cervical status is likely to be perceived as having low-risk of cesarean delivery. Increasing maternal age and gestational age may both be perceived as increasing the chance of fetal intolerance of labor and fetal size, and thus increasing the risk of cesarean delivery. Obesity is typically perceived as being associated with increased fetal size, and decreased amniotic fluid with higher rates of fetal intolerance of labor and subsequent cesarean delivery.

Most clinicians utilize these perceptions to adjust the a priori risk of cesarean delivery. Unfortunately, most individuals are exceptionally poor at such adjustments.153 Such modifications to a priori risk are known as Bayesian updating, in which one considers the joint probability of two events, in this case, induction of labor and cesarean delivery, occurring together. Some adjustment on the relationship is then performed to provide an updated, posterior estimate of a prior probability. Further, even if adjusted accurately, the overall sense of a patient with an increased risk of a failed induction leads to increased pessimism regarding her chances. Since clinicians make the diagnosis of an indication for the cesarean (e.g., fetal intolerance of labor, active phase arrest, cephalo-pelvic disproportion), clinicians may be more likely to make these diagnoses in women whom they believe are more likely to fail during their induction of labor. This introduces diagnostic bias that tends to bias results away from the null. Thus, if these factors are associated with increases in cesarean delivery, such diagnostic bias would only amplify these findings, making the difference in risk appear larger than the truth. Such findings, in turn, may become accepted into the understanding and practice of obstetrics, further biasing clinicians in a cyclic fashion.

How Successful Induction of Labor was Defined

Since the purpose of induction of labor is to cause a non-laboring woman to go into labor, a reasonable working definition is achieving active labor as a measure of success. However, of the 80 studies included, only one of the studies utilized this definition. The most commonly utilized method of defining successful induction in our included studies was achieving any vaginal delivery [46/80 (58.5 percent) of studies]. Considering the effects of elective induction of labor, this metric is a reasonable way to examine the effect of induction of labor. If induction of labor leads to achieving active phase of labor in all attempts, yet eventually leads to a higher rate of cesarean delivery, the downstream effects of induction need to be considered. However, one problem with utilizing cesarean delivery as a metric is the wide variation in physician practice and the potential bias introduced by a physician's beliefs about the effect of induction of labor on cesarean delivery and other perinatal outcomes. One way to minimize such bias is by examining the indication for cesarean delivery, which was done in the largest study we reviewed.27 However, even the indication for cesarean delivery is subject to physician influence, as some clinicians may define indications for cesarean delivery, such as active phase arrest, cephalo-pelvic disproportion, and fetal intolerance of labor, differently based on whether the patient was being induced or in spontaneous labor.

Several studies used a variety of time-based definitions of a successful induction: Vaginal delivery within 6, 12, 18, or 24 hours or active labor within 12 hours. These definitions are particularly applicable when considering the cost-effectiveness of elective labor induction. Labor and delivery units are nearly as costly to function as intensive care units. A considerable portion of the high cost is attributed to the high ratio of nurses to patients as well as to the use of costly supplies. When considering a policy of offering or recommending elective induction of labor from a societal standpoint, consideration must be given to the amount of additional time such women will be required to spend on the labor and delivery unit compared to those who undergo spontaneous labor. If this amount of time equals 12–24 additional hours and 25 percent of women are induced, the societal cost may be in the billions of dollars annually.

Another important aspect of the time-based definition of induction of labor is how it influences the culture of both providers and patients when considering what is defined as a “successful” induction of labor. For instance, if achieving active labor within 12 hours is the threshold for a successful induction, and those who do not achieve this are considered failures, might these perceived failures be more likely to proceed with a cesarean delivery? In a small study of the predictive value of the amount of time spent in the latent phase subsequent to cesarean delivery, Simon and Grobman demonstrated that women who progressed into active labor within 12 hours had a 67 percent to 86 percent chance of achieving vaginal birth, whereas women who reached active labor after 18 hours proceeded to a vaginal birth only 31 percent to 33 percent of the time.10 However, this brings two considerations to light. First, women who reached the active phase of labor later may have been perceived as failures, and thus their clinicians may have been more likely to proceed to a cesarean delivery. Second, if a third of the women whose latent phase of labor extended beyond 24 hours were still able to achieve vaginal delivery, is there harm in continuing the labor in this setting? These investigators examined some of the associated outcomes with prolonged latent phase of labor, but were underpowered to make consistent conclusions regarding the potential risks.

In summary, it appears that each of these measures, achieving active labor, mode of delivery, and length of time to achieve each of these outcomes, have validity as metrics. Thus, future studies should attempt to capture all of them.

Decision Analytic Outcomes

In our exploratory models among theoretical cohorts of nulliparous women at 39, 40 or 41 weeks gestation, induction of labor was cost effective and led to improved maternal and neonatal outcomes compared to expectant management of pregnancy. These findings were extremely robust at 41 weeks of gestation. However, at 40 and 39 weeks of gestation, while the baseline model was cost-effective, but the findings did not stand up to multivariate sensitivity analyses. The improvement in neonatal outcomes is expected, as the outcomes examined (intrauterine fetal demise, macrosomia, shoulder dystocia, meconium stained amniotic fluid, meconium aspiration syndrome) are all known to increase with gestational age.87, 98 In our models, we found that 96 percent of the benefit in the cost-effectiveness analysis was from reduced IUFD. Since induction of labor at term will always lead to a lower gestational age at delivery, this option will necessarily lead to reductions in these outcomes. Furthermore, when considering maternal outcomes, if a woman undergoes induction of labor, she is no longer at risk for the pregnancy complications associated with continuing gestation, such as preeclampsia. Therefore, these complications will also be reduced. In addition, those maternal outcomes that are dependent on changes in fetal status, such as the increased risk of severe perineal laceration associated with delivery at a later gestation, will also necessarily decrease with a policy of induction of labor. These changes in outcomes are both intuitive and clinically relevant to comparing induction of labor to expectant management of pregnancy.

Acknowledging that these clinical outcomes are virtually all improved with a policy of induction of labor, the most easily calculable potential disadvantage of induction (and the theoretical concern that is most often stated by opponents of labor induction) is an increased risk of cesarean delivery, and an increase in costs associated with induction of labor. Review of the literature for this project and other existing meta-analyses90, 91 indicate that induction of labor may be associated with a decreased risk of cesarean delivery. Depending on the reduction of risk associated with induction, the cost effectiveness of this intervention increases. Importantly, though, even if this assumption is reversed, and induction of labor is assumed to lead to a 22 percent increase in risk of cesarean delivery, induction remains a cost-effective intervention at $27,021 per QALY when examining the 41 week cohort, $27,397 per QALY when examining the 40 week cohort, and $71,945 when examining the 39 week cohort. Of note, induction of labor is not cost-effective when it leads to more than a 50 percent increase in cesarean delivery at 41 weeks, more than a 40 percent increase in cesarean delivery at 40 weeks, and more than a 25 percent increase in cesarean delivery at 39 weeks of gestation. Biologically, we do not believe that induction of labor should lead to such increases in cesarean delivery based on the current review; however, it is difficult to determine from this review how induction of labor will truly affect cesarean delivery rates in actuality given the other pressures on clinicians to perform a cesarean delivery such as scheduling of outpatient clinical time and medical-legal considerations related to labor and delivery management. Further, we did not consider downstream effects on future pregnancies. If induction of labor leads to higher rates of cesarean, future pregnancy outcomes would be worsened as well.

Univariate sensitivity analysis revealed that both the 40 and 41 week models were robust, remaining cost-effective over a wide variation in individual inputs, while the 39 week model was more sensitive, particularly to the risk of cesarean delivery, as discussed above. However, as noted above, the 40 week model was less robust when multivariate sensitivity analysis was conducted, underscoring the importance of conducting such analyses. Most interesting were the situations in which induction became the dominant strategy; this occurred only in the 40 and 41 week models. In the 41 week model, this occurred when the rate of cesarean delivery in the induction group was less than 70 percent of the rate in the expectant management group or when the likelihood of spontaneous labor at 41 weeks dropped below 25 percent. In the 40 week model, this occurred when the rate of cesarean delivery in the induction group was less than 82 percent of the rate in the expectant management group or when the likelihood of spontaneous labor dropped below 20 percent. These characteristics help define two different populations that may benefit from induction of labor. The first is those women who are “easy” to induce, incur very little risk of cesarean delivery with induction, and gain the benefits of delivery at an earlier gestational age. The second is a group with the least likelihood of spontaneous labor in the following week benefiting the most from induction of labor, as without induction, they are highly likely to remain pregnant and thus potentially incur the negative outcomes associated with advancing gestational age.

Theoretically, those with the lowest likelihood of spontaneous labor might have an increased risk of failed induction of labor or require additional resources for induction of labor, as characteristics such as multiparity and a favorable Bishop's score have been associated with successful induction as well as spontaneous labor. This relationship was further explored in bivariate sensitivity analysis first examining the effect of varying the relative risk of cesarean delivery with induction and the probability of spontaneous labor, and then the cost of induction of labor and the likelihood of spontaneous labor. For both the 40 and 41 week cohorts, induction was still cost-effective for the population of women with the higher than average likelihood of spontaneous labor as long as the relative risk of cesarean delivery remained close to 1, but the absolute likelihood of spontaneous labor at an earlier gestational age had to be lower in order to maintain the same threshold of cost effectiveness.

Conceptually, the issue of women who have the lowest chance of going into labor in the following week potentially reaping the greatest benefits of induction of labor has not been particularly well explored in the literature. Generally, cohort studies of elective induction of labor have been of women who have favorable cervices31 because it is presumed that the increase in cesarean delivery will be the least in these women. Further, randomized trials of elective induction of labor at 41 weeks and earlier often include only women who have an unfavorable cervix because of the assumption that elective induction of labor is reasonable in those with a favorable cervix.27, 74 However, because ongoing risk leads to higher rates of complications in the expectant management group, those women at the lowest risk of going into spontaneous labor in the following week are those who theoretically may benefit the most from induction of labor; women with unfavorable cervices or those who are at increased risk of progressing to a postterm pregnancy, such as nulliparas or obese women.154

Extending these concepts to the 39 week cohort is dependent on the comparison strategy chosen. While induction remains a cost effective strategy in many scenarios, if the alternative strategy is induction of labor at 40 weeks, then variation in the cesarean delivery rate, cost of induction, and probability of spontaneous labor all impact cost-effectiveness. The complications of term and post-term pregnancy are increasing at this point, but the slope is not as steep as it is from 40 weeks and beyond; thus, induction at 40 weeks becomes the more cost-effective option in some scenarios. Again, this highlights the idea that the women who are at greatest risk of postterm pregnancy are the ones that will benefit most from induction; the challenge is how to identify this cohort at 39 weeks.

One methodologic issue of decision and cost-effectiveness analysis that we explored deserves mention. It is unclear in models of pregnant women whether and how to include the utilities related to the neonate.155 In general, decision-analytic models have not utilized preferences of family members (e.g., partners, parents, siblings, or children), though the utilities of these individuals would certainly be affected by changes in the health status to the individual being studied. Thus, one way to design these models would be to simply consider the utilities of the pregnant woman herself. However, because the decisions being made directly affect the neonatal outcomes as well, we believe that preferences related to the neonate should be directly included in the decision and cost-effectiveness models in such cases.156 In our baseline analysis, we incorporated both maternal and neonatal QALYs. However, to investigate the effect of such inclusion, we also conducted analyses excluding the neonatal valuations. In the 39, 40 and 41 week models, the inclusion of neonatal QALYs is necessary to make the model cost effective as compared to expectant management. In fact, with the utility estimates that we were able to incorporate, avoidance of intrauterine fetal demise by delivery at an earlier gestational age drives the results of the model. Future decision analytic and cost-effectiveness analyses should consider the effects of both maternal and neonatal QALYs as well as how the model is affected by excluding neonatal QALYs. To be clear, the largest driver of QALYs in our model were the loss of both maternal and neonatal utilities from IUFD.

Costs were another area of exploration in our sensitivity analysis to which the cost-effectiveness of the model had some variation. Examining the impact of cost, at 41 weeks, if the likelihood of spontaneous labor is low, even if the additional cost of induction of labor increased to 400 percent of baseline, induction remained a cost-effective intervention. The 40 week model was more sensitive to the cost of induction of labor, although still cost effective up to 200 percent of the baseline cost estimate for those with the lowest likelihood of spontaneous labor. This trend continued in the 39 week model, taking into account a lower baseline spontaneous labor rate. At all gestational ages, this defines the population of people with the lowest likelihood of spontaneous labor as the ones most likely to benefit from induction of labor, even if additional cost is incurred during induction.

We believe that the utilization of decision analysis to characterize the expected outcomes in a population of pregnant women who would undergo either induction of labor or expectant management of the pregnancy at or beyond a particular gestational age enhances this literature review of elective induction of labor. Acknowledging that no model is able to fully capture the complexity of a clinical situation, or include all of the factors that a clinician integrates, decision analysis provides another perspective on the limited information currently available regarding elective induction of labor.

Elective Induction of Labor—From Evidence to Actual Practice

While elective induction of labor seems like a promising intervention to simultaneously reduce the cesarean delivery rate as well as prevent a variety of term complications of pregnancy, there are a number of practical considerations which must be addressed. First, generalization from the existing evidence to clinical practice today is problematic. While the cesarean delivery rates in many investigations summarized in this review range from 10 to 20 percent, the rate of cesarean delivery in the setting of induction of labor in the U.S. in 2003 at 41 weeks of gestation was 27 percent. In fact, the overall cesarean delivery rate in the U.S. has risen from 5.5 percent in 1970 to reach its highest level yet in 2006 of 31.1 percent,82, 157, 158 despite a goal for the primary cesarean delivery rate from Healthy People 2010 of 15 percent.159 Why is the cesarean rate increasing? One possible reason for the rise in the cesarean delivery rate is that there may be a rise in the need for cesarean. Two possible mechanisms that may contribute to the increasing need for indicated cesarean are increasing birthweight160 and increasing maternal obesity and weight gain.118, 152, 161 Another possible reason may be a rise in elective cesarean delivery by maternal request (CDMR).162 The topic of CDMR is currently of heightened interest leading to a recent NIH State-of-the-Science conference in March, 2006. The statement from this meeting concluded that future research was necessary to examine both the “current extent of CDMR and attitudes about it.”163 Another potential mechanism contributing to an increasing rate of cesarean delivery are the incentives that providers face. As noted in the introduction, these incentives are not simply reimbursement. Specifically, the time costs that providers in private practice face when deciding to proceed with expectant management of a labor in progress versus proceeding with a cesarean delivery are high; it takes less time to perform a cesarean delivery now as compared to expectantly managing a dysfunctional labor. This is likely to vary based on practice setting. For example, in a practice setting which incorporates the use of laborists, practitioners dedicated to care in the labor and delivery unit (similar to hospitalists in internal medicine), being patient during an induction of labor has far less economic or time pressure on the practitioner. Alternatively, for clinicians who are charged with both in house obstetric care and simultaneously are providing care in the outpatient setting, there are both economic and time pressures to minimize the length of labor whether through augmentation or, in some cases, cesarean delivery. Further, there are additional medical-legal pressures, particularly in states without “pain and suffering” caps where recent settlements of greater than $20 million have been awarded. A clinician deciding to proceed with a labor that has veered ever so slightly from what is perceived as normal may believe he or she is at risk for liability.164

These incentives may lead to decreased patience on the part of providers. Such patience is essential if a goal of reducing the cesarean delivery rate is considered important. For example, a common indication for cesarean delivery is active phase arrest, which is frequently defined as absence of cervical change for two hours in the active phase of labor in the presence of adequate uterine contraction. Rouse et al. reported a prospective cohort study in which clinicians waited for an additional two hours after the diagnosis of active phase arrest, and 60 percent of these women initially diagnosed with active phase arrest went on to achieve vaginal delivery.165, 166 Myers et al validated these findings, but found a reduced success in the setting of induction of labor.167 As reported above by Simon and Grobman, if clinicians extended the definition of prolonged latent phase in the setting of induction of labor to 18 hours, one would achieve vaginal delivery in over 60 percent of women being induced; and if such a threshold was extended beyond 24 hours, another third of these women would deliver vaginally. Given that such patience may lead to lower cesarean delivery rates, changing the financial incentives to clinicians to reimburse at a higher rate for vaginal delivery is one possible solution to encourage a lower rate of cesarean deliveries.

Thus, the question of whether elective induction of labor is supported as a reasonable intervention is implicitly tied to how it will actually affect both rates of cesarean delivery and perinatal outcomes. Since many of the maternal outcomes are tied to the risk of cesarean delivery, there might be an increase in a number of patient complications if the cesarean risk actually rises. Although, in our decision-analytic model where we utilized an increase in cesarean delivery of 22 percent from induction of labor, we found that on balance, short-term maternal outcomes were similar and neonatal outcomes were better. Despite this, the long-term impact of a rising cesarean delivery rate may be revealed in subsequent pregnancies which we did not include in this model.

It is essential to determine not only how elective induction of labor will affect outcomes in the research setting, but particularly in private practice and community hospitals where the majority of women receive their care. In most of these settings, we do not currently have adequate data structures to examine the effect even in observational studies. Thus, concomitant with a large prospective RCT to examine elective induction of labor, it is important to collect specific data on elective induction of labor and the wide variety of potential confounders and perinatal outcomes at a population level. Further, the importance of exercising patience when managing labor and delivery should be emphasized in the teaching of current obstetrician-gynecologists-in-training to provide a long-lasting impact on the cesarean delivery rate in the future.

Limitations of This Report

When conducting a systematic review, there are limitations based on particular MeSH terms utilized, inherent shortcomings in the existing literature, and the heterogeneity of the existing data. Cost-effectiveness analyses suffer similar data limitations and are further limited by potential design flaws. We discuss below the limitations of our systematic review and cost-effectiveness analysis.

Limitations of the Review

When examining the limitations of a systematic review, consideration of the limitations of the search strategy utilized, limitations of the databases searched, and technique of data abstraction is necessary. Regarding the search strategy employed, we believe we captured a great majority of the literature on elective induction of labor. However, in recognizing the potential for missing studies, we chose ten known studies of elective induction of labor and made sure our search strategy captured all of them. Further, once the initial review was completed, we searched bibliographies of included studies for additional studies on elective induction of labor. We only searched the MEDLINE database, but with reviewing the bibliographies, we believe that we captured nearly all of the elective induction of labor literature.

One aspect of the search strategy that limited 100 percent identification of the literature of interest was exclusion of non-English language studies. As noted earlier in this report, one of the largest RCTs on induction was published in French.168 However, given time and budgetary limitations, it was not feasible to translate non-English studies. Having reviewed other related meta-analyses, we believe this is the only pertinent non-English study we did not identify in our search.

Our review and abstraction was also challenging. While we utilized dual review and resolution of disagreements, much of the older literature was difficult to interpret with respect to study design and control groups.

Limitations of the Existing Evidence

The body of evidence is limited by the relative paucity of studies, the small number of well-designed studies, the number of adequately powered studies, the breadth of reported outcomes in these studies, and the analytic design. Examination of heterogeneity often revealed that the studies collected were dissimilar, thus, we could not synthesize the data into a single summary statistic. Additionally, several of the outcomes and predictors may have been subject to publication bias which is concerning for overstating our findings.

The literature identified and included in this review was distributed broadly across countries and spanning a wide time period. Cesarean delivery, as a principal outcome of interest, was a concern as cesarean delivery rates are sensitive to the cultural context and have demonstrated dramatic change over time, particularly during the last three decades. Thus, a study conducted in one decade may not necessarily inform practice in another decade with respect to cesarean delivery. Further, because most of the studies did not stratify analyses or examine for interaction or effect modification, it is uncertain whether the overall effects observed apply to all subgroups of the population.

Inadequate quantity of evidence. The overall quantity of studies was somewhat limited. For the vast majority of outcomes, fewer than five studies reported data. Synthesis of the literature with few studies becomes challenging as a single study may significantly affect the overall outcomes and introduce heterogeneity. This limited the ability to perform stratified analyses or use multivariate techniques such as metaregression. In addition to the small number of studies, many of the available studies were small to medium in size, thus further limiting the overall power of the review. Finally, the breadth of reported variables of interest was quite limited. The majority of studies only report a handful of outcomes. This reduced the availability of data that was affected by the limited number of studies, further limiting the power.

Inadequate study design. As mentioned in the discussion, there were challenging issues regarding study design. While the majority of the RCTs were properly designed to compare elective induction of labor to expectant management of pregnancy, three identified studies used an incorrect analytic design: Excluding women who were induced while allocated to the expectant management arm. With the exception of one study,31 all of the cohort and case-control studies utilized an inappropriate control group, comparing elective induction of labor to spontaneous labor, rather than expectant management. However, even in this study, there was considerable risk for potential confounding, as the women who were electively induced all had favorable cervices. Other concerns with study design included a lack of careful consideration of gestational age. While the studies examining induction of labor at 41 weeks of gestation as compared to expectant management were generally specific with respect to gestational age, the studies before 41 weeks of gestation did not have specific randomization arms at 39 and 40 weeks of gestation. Thus, generalizing the information from the synthesized summary statistic to gestational ages prior to 41 weeks is not reasonable with the available data.

Inadequate adjustment for confounding. For most of the cohort and case-control studies there was inadequate adjustment for potential confounding. The majority of studies did not utilize either multivariable techniques or stratified analyses to begin to adjust for potential confounders. Unfortunately, even, in recent studies which attempted to control for bias, many studies were too small or contained too little information on particularly important known confounders to make appropriate adjustments. Finally, since several of the most important confounders, such as local practice styles, local cesarean rates, and provider characteristics were not measured in any of the studies, adjustments could not be achieved, leading to potential confounding and the diagnostic bias discussed above.

Limitations of Decision Analysis

We used models to represent clinical scenarios, which can be limited in scope and miss a number of intangible factors that can be realized in a clinical study. The size of a model and incorporated outcomes depends on clinical expertise and the balance of too great a burden of detail and too little can be a challenge. While a more complex model may better approximate the true clinical picture, its complexity may obfuscate the identification of what the key inputs and outcomes are in a more simplistic model. Thus, we did not include every possible maternal and neonatal outcome; rather, we incorporated a number of the more severe outcomes, specifically those that may be impacted by elective induction of labor, advancing gestational age, and mode of delivery.

Once a decision-analytic model is created, point estimates of probabilities must be identified in the existing literature in order to populate the model and allow for prediction of outcomes. We incorporated probabilities from a variety of sources for the current model, some of which may lack sufficient power to support the accuracy of the generated point estimate. However, in settings such as this, the strength of decision analysis is to consider a wide confidence interval around unsure estimates. We created univariate sensitivity analyses around every point estimate in the model and generally found our results to be robust. Further, we conducted a Monte Carlo simulation which allows for a large number of trials to be performed, each of which samples the probabilities at each probability node and then predicts which path will be taken. Such a simulation models the path of a clinical trial and provides results in a similar fashion. Such sensitivity analyses allow for extremely uncertain data to be included in the model and to allow for close and accurate examination of how poor data would impact the study conclusions.

More than just probability estimates, the utility estimates used in this model were abstracted from studies which did not directly address what these preferences represent and how they are utilized in the current model. For example, maternal preferences towards neonatal outcomes such as neonatal death, intrauterine fetal demise, and cerebral palsy, were estimated from studies by Kuppermann et al.,140 which examine outcomes such as pregnancy loss and Down syndrome. Generalization of these data is questionable and may provide bias with respect to the decision analytic and cost-effectiveness results. However, our results were robust to sensitivity analyses, and without generalization of these data, only perinatal mortality could have been addressed. It is not feasible to compare the multitude of perinatal outcomes or to characterize the quality of life issues related to these outcomes.

Finally, there are limitations in the existing cost data utilized for the cost-effectiveness analyses. Much of the data utilized is more than 10 years old. This is a concern in health economics studies since health care costs are rapidly increasing. While the medical component of the consumer price index can be used to project the older costs forward, this is a measure of overall medical cost increases rather than the how specific costs are increasing and being measured. Further, a number of the costs utilized were estimated from the existing literature and generalized for use in this study. These costs may not have been estimated based on the specific clinical situations being described in the current study. While we conducted sensitivity analyses over wide ranges of these cost inputs, better cost data in this area would certainly facilitate more accurate estimates of the cost-effectiveness of elective induction of labor.

Future Research Considerations

We order this section by Key Questions, similar to other sections in this report. Key Questions 1 and 2 are part of the same research agenda, comparing elective induction of labor to expectant management, thus the first section will discuss these two mutually.

Elective Induction of Labor Versus Expectant Management of Pregnancy

When addressing issues involving elective induction of labor, one must consider the intended goal. Similar to the commentary in the AHRQ report on cesarean delivery by maternal request, (CDMR) which noted that since women may go into labor and deliver via one of three modes of delivery (a spontaneous vaginal delivery, operative vaginal delivery, or cesarean delivery), one must consider planned or intended modes of delivery.107 In the setting of elective induction of labor, the comparison group, which consisted of women whose pregnancy were expectantly managed, can experience either spontaneous labor, or subsequent development of complications of pregnancy that requires induction of labor. Further, these potential outcomes (i.e., spontaneous labor, complications of pregnancy, or induction of labor) can occur at any point in the future at a wide variety of gestational ages. It was surprising that even when prospective RCTs were appropriately designed, several authors analyzed the data by comparing induction of labor to spontaneous labor rather than induction of labor to expectant management as intention to treat. In both RCTs and observational studies, strict use of the appropriate control group, women being managed expectantly, is important.

Outcomes measured. In studies of elective induction of labor compared to expectant management, the focus should be on consistently reporting a wide variety of perinatal outcomes. While we anticipated examining a wide range of outcomes, in reality we obtained information only on a few and were able to synthesize information only on a handful. With respect to mode of delivery, the outcomes, cesarean and operative vaginal delivery, were usually recorded. However, to further determine the effect of labor induction on specific modes of delivery, it would be beneficial to report the indications for both cesarean delivery and operative vaginal delivery. In particular, if a “failed induction” is the indication for cesarean delivery, it would be helpful to report the number of hours involved in the attempted labor induction, and the methods (e.g. prostaglandins, Foley bulb, oxytocin, AROM) utilized, as well as the timing of these methods relative to different phases/stages of labor. Further, since there is some evidence regarding induction and augmentation of labor and fetal position,79, 80 which, in turn, is associated with mode of delivery, fetal position should be recorded as an outcome.

Other maternal outcomes which should be routinely reported in studies of elective induction of labor include the following: Estimated blood loss, incidences of postpartum hemorrhage, blood transfusion, chorioamnionitis, endomyometritis, perineal lacerations, epidural use, length of hospital stay, as well as uncommon but severe morbidities such as pulmonary embolus, amniotic fluid embolus, hysterectomy, and mortality. Since these outcomes are both more severe and less frequent, it is difficult to garner sufficient power to evaluate in a single prospective RCT; thus larger health system or birth certificate data could include elective induction of labor, and large cohort studies could potentially accurately quantify these complications. Long term outcomes such as subsequent fertility, subsequent placentation, subsequent mode of delivery, and pelvic floor injury as represented by urinary incontinence, fecal incontinence, and pelvic organ prolapse should also be examined.

Neonatal outcomes that should be reported routinely in studies intending to examine the effects of labor induction include the following: Umbilical artery blood gases, 5-minute Apgar score, particularly 5-minute Apgar less than 4, respiratory distress syndrome, transient tachypnea of the newborn, presence of meconium-stained fluid, meconium aspiration syndrome, neonatal sepsis, admission to intensive care nursery (ICN), shoulder dystocia, birth trauma including brachial plexus injury, facial nerve palsy, skull fracture, other fractures, cephalohematoma, subgaleal hemorrhage, intracranial hemorrhage, hyperbilirubinemia, birthweight, IUGR, macrosomia, hypoglycemia, polycythemia, length of stay, breastfeeding, and mortality (antepartum, intrapartum, and neonatal). Long-term outcomes such as infant and childhood outcomes of behavior and intelligence should also be assessed. Similar to maternal outcomes, due to the low incidence rate of these outcomes, even large prospective trials are not adequately powered to assess these outcomes. If properly designed and well executed, large cohort studies may potentially overcome limited power and some of the inherent flaws of observational studies, potentially offering vital information to elucidate the rate of these outcomes in association with induction of labor.

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. Qualitative studies of how women perceived their birth experience in the setting of elective and indicated induction of labor, how they felt their preferences were incorporated into the decision-making process, whether they felt pressured by providers to choose one clinical path or another, how they were counseled and consented, and how their birth experience affected their perceptions of quality of life in future pregnancies all need to be conducted. Specific quantitative measures of patient quality of life would also contribute to the discussion. Both measures of pain and utility on labor and delivery as well as quality of life measures throughout the short- and long-term postpartum periods would inform the understanding of how individuals perceive this intervention. Interestingly, while elective induction of labor allows for some control as to when labor will begin, it also may take the management of early labor out of the parturient's control. How women perceive this intervention will greatly inform the discussion regarding whether it should be routinely offered, and how it might be best conducted to optimize perinatal outcomes and maintain patient satisfaction. Specific economic measures such that micro-costing all of the labor, supplies, time costs, and overhead costs of the induction of labor experience should be examined. When determining how to use societal dollars to facilitate better health outcomes, allocation of these scarce resources cannot occur without reproducible estimates of these costs. In addition, these economic and quality of life issues should be estimated in subsequent pregnancies as these are affected by prior experiences and outcomes on labor and delivery.

The outcomes mentioned above can happen to either group of patients, but the expectant management group is at risk of developing other complications of pregnancy that occurs only in the setting of prolonging gestation. Such pregnancy complications that will arise in the expectant management group should also be recorded including: Preeclampsia, cholestasis of pregnancy, abnormal antenatal fetal testing, placenta abruption, oligohydramnios, intrauterine fetal growth restriction (IUGR), induction of labor, and intrauterine fetal death (IUFD). Knowing these specific risks of complications will further assist in the ongoing evaluation of the risks and benefits of elective induction of labor versus expectant management.

Studies examining this litany of outcomes would need to be adequately powered. For example, for a relatively common outcome such as cesarean delivery, if we assume a baseline rate of 20 percent, a study would need 400 women in order to have 80 percent power to identify a 50 percent difference with a two-sided alpha of 0.05 and 532 women for 90 percent power (Table 4.2). However, when examining less common outcomes, such as a rare neonatal morbidity at a prevalence of 1 percent, a study would need 9,346 women to have 80 percent power and 12,506 women to have 90 percent power.

Table 4.2. Sample size estimates for prospective trial of elective induction of labor as compared to expectant management of pregnancy.

Table 4.2

Sample size estimates for prospective trial of elective induction of labor as compared to expectant management of pregnancy.

Thus, to accomplish a well-designed, prospective RCT would likely require a multi-center approach, for example the Maternal-Fetal Medicine Units (MFMU) network, to accomplish such a large study. While one may appropriately address some of these issues using large observational data, prospective studies will need to be conducted to truly answer the causal effect of these questions.

Study Design

The majority of RCTs examining induction of labor versus expectant management of pregnancy have been conducted in women who are considered postterm or at least prolonged (greater than 41 weeks of gestation).90 In our review, there were only five RCTs that compared induction of labor to expectant management at the design phase and only three that did so in the analytic phase. One of these studies168 was excluded from our analysis because the primary language of the paper was not English. However, even with its inclusion, these studies are heterogeneous with respect to study design, analytic choices, outcomes reported, and gestational age at randomization of the patients. This last issue is of paramount importance in any future study is conducted. There are two choices for the sequence of studies to be conducted. The first approach would be to conduct the studies incrementally. In this scenario, the study would preferably be an RCT of induction of labor at 40 weeks gestation as compared to expectant management, followed by an RCT at 39 weeks gestation with similar comparison groups. Alternatively, a three-armed study would be an RCT at 39 weeks gestation versus expectant management and within the expectant management arm, those who achieve 40 weeks of gestation would then be subsequently randomized to induction of labor at 40 weeks of gestation versus induction at 41 weeks gestation. While the latter study is larger and more expensive than each of the single studies, it is likely less expensive overall and addresses induction at 39 versus 40 weeks as well as 40 versus 41 weeks. A subsequent study of induction of labor at 38 weeks of gestation versus expectant management would not meet standard of care issues set forth by the ACOG88 because of concerns regarding neonatal outcomes in elective inductions prior to 39 weeks. However, as more data on neonatal outcomes at 38 versus. 39 weeks of gestation are published, such a distinction may evolve.

Stratified Randomization

Since nulliparous and multiparous women carry such different risks of cesarean delivery, they need to be randomized independently. In all practicality, two separate studies would be carried out. A similar concern applies to the cervical status. Because of the current dogma regarding elective induction as appropriate in women with a favorable cervix and more problematic in those with an unfavorable Bishop score, it would likely be best to randomize these women separately as well. Doing so, of course, will significantly increase the overall sample size if the analytic plan is to examine these subgroups independently. Further stratification would likely prove too onerous to accomplish. But as we begin to understand more about risk factors, such as obesity,154 for prolonged, and postterm pregnancy, these factors certainly should be considered and examined closely as potential confounders.

Data Analysis

For the prospective RCTs, traditional data analysis seems adequate. Surveillance of important confounders as mentioned above, including parity, cervical status, BMI and obesity, race/ethnicity, co-morbid medical conditions and complications, is important and controlling for potential confounders with multivariable logistic regression models should be utilized. Further, stratified analyses, particular by parity and cervical status, but also by other potential confounders will aid in characterizing the effects regardless of these confounders. Also, examination for interaction and effect modification will help to determine whether there are particular subgroups that may benefit more or less from elective induction of labor.

For observational studies, these same issues regarding confounding bias and how to address such methodologic challenges using statistical techniques are even more important. More so, constructing appropriate control groups to simulate clinical scenarios such that elective induction of labor is compared to expectant management is tantamount. Utilizing the appropriate study design and data analysis will be a step towards characterizing the potential benefits or harms of elective induction of labor.

Predictors of a Successful Induction of Labor

It is not surprising that multiparity and a favorable cervix have been examined frequently in the literature and are found to be associated with greater success in the setting of labor induction. However, there are many other factors that may be associated with a successful induction which have not been properly examined. Such factors include: Maternal demographics such age, weight, height, BMI, race/ethnicity, socioeconomic status as well as obstetric and medical history such as spontaneous or therapeutic abortion, uterine fibroids, chronic hypertension, diabetes, and preeclampsia. Further, systems and care issues such as provider characteristics, day of the week, time of day, volume of the obstetric unit, and overall cesarean delivery rate on the obstetric unit should also be examined as potentially important predictors of induction of labor success.

Definitions of a Successful Induction of Labor

The definition of a successful induction of labor can alter the perception of elective induction of labor. If defined in terms of cesarean delivery, an elegant comparison of success to that of expectant management is achieved. If defined with respect to time interval required to achieve active labor, while this is a useful measure when approximating the economic impact, it certainly is unreasonable when comparing elective labor induction to expectant management since more of the women expectantly managed will experience spontaneous labor. Furthermore, the sociocultural effects of defining the success of an induction of labor by the amount of time it takes to achieve active labor or vaginal delivery should be examined as well. In settings where such time thresholds are utilized, does this lead to an increase in unnecessary cesarean deliveries simply because of a lack of patience by both the providers and patients? Comparative work examining these effects between providers and obstetric units is certainly important when attempting to create guidelines that might inform the practice of obstetrics as a whole.

Conclusions

In this systematic review and decision analysis of elective induction of labor, we found that overall elective induction of labor as compared to expectant management of the pregnancy was associated with an approximately 20 percent reduction in the rate of cesarean delivery and a 50 percent reduction in the presence of meconium in the amniotic fluid. However, the majority of these studies were just in women at or beyond 41 0/7 weeks of gestation; prior to 41 weeks of gestation, there was insufficient evidence from the review to address these outcomes. These findings are consistent with other meta-analyses of induction of labor in postterm and term pregnancies, but are contrary to many observational studies. The existing literature is not powered to examine many of the other complications of pregnancy; however it is assumed that a number of complications must be reduced by elective induction of labor, simply because pregnancy complications such as preeclampsia or IUFD can no longer occur if the pregnancy is ended by induction of labor. These findings were reflected in the results of our decision-analytic models. Further, when we incorporated costs into the models, it appears that elective induction of labor is a cost-effective intervention at 41 weeks of gestation and may potentially be so at earlier gestations. These results prior to 41 weeks of gestation require further examination in a large, prospective randomized trial before routine adoption into clinical practice. Further, because of the heterogeneity in the management of labor induction, which varies widely between providers and institutions, careful examination of the impact of such policies in a wide variety of 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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