Induction of labor is a common obstetric intervention, now performed in 22% of all births.1 The effect of induction of labor on the duration and complications of labor is unclear.2 Some observational studies suggest that induction increases the risk of complications such as postpartum hemorrhage, operative deliveries, and cesarean deliveries.3,4 Other prospective studies suggest that for women with a postdate pregnancy, induction may not increase these risks and may even lower the risk of cesarean delivery and adverse fetal outcomes5–7; less is known about the effect of induction in other populations.
The second stage of labor is of particular interest because the optimal duration of the second stage is a subject of current debate. Although allowing for a longer second stage increases rates of vaginal delivery, the incidence of maternal complications such as chorioamnionitis and perineal tears increases with each hour of second-stage duration.8–10 In addition, certain maternal factors such as parity and epidural use appear to change the average duration and complication rates of the second stage.11,12 It is not clear whether induction of labor also has an effect.
Our study sought to compare the length and complications of the second stage of labor between women who reach full dilation after induced or spontaneous labor.
MATERIALS AND METHODS
We undertook a large retrospective cohort study to examine the effect of induction of labor on the duration of the second stage, mode of delivery, and delivery complications.
At our institution, a tertiary care center, prenatal care and deliveries are performed by obstetricians and midwives who are part of a single large faculty practice and deliver at a single hospital. Patients receive prenatal care at an outpatient clinic located on the main hospital campus or at community health centers in the surrounding areas. The community health centers were established to improve access to health care for the medically underserved; health center patients are primarily low income and publicly insured or uninsured. Elective inductions are discouraged by the practice and are audited regularly. Although this group shares protocols and guidelines on indications and timing of induction, there is no standardization of specific management on labor and delivery such as timing of cervical examinations during labor.
The obstetric electronic medical record at our institution is used for clinical and research data collection and provides clinical and demographic information that prospectively details the events of pregnancy through the early postpartum period. After obtaining Institutional Review Board approval from the Partners Human Research Committee, a database consisting of information from the obstetric electronic medical record was queried to identify all women with nonanomalous viable singleton vertex gestations greater than 37 weeks of gestation who reached full dilation from January 2001 through June 2009. Prenatal and demographic data obtained included maternal age, maternal height and weight, medical history (which is entered into the obstetric electronic medical record during prenatal care), and best estimated date of confinement (based on last menstrual period unless ultrasonography corrected the date by more than 1 week in the first trimester or 2 weeks in the second). Gestational age at delivery is generated based on this best estimated date of confinement. Delivery information, which is entered by the clinicians caring for the patient at the time of delivery, included: whether labor was induced, indication for induction, labor analgesia, cesarean analgesia, whether full dilation was reached, time of reaching full dilation, time of delivery, Apgar scores, birth weight, and presence or absence of delivery complications (shoulder dystocia, chorioamnionitis, postpartum hemorrhage). Delivery information is routinely entered within 2 hours of delivery and is reviewed by administrative staff within 24 hours of delivery.
Length of the second stage was calculated as the number of minutes from the first cervical examination that revealed full dilation until delivery. In our study, prolonged second stage was defined as more than 180 minutes, which is the American College of Obstetricians and Gynecologists definition of prolonged second stage for nulliparous women with an epidural.13 We chose to use this same definition for all women because multiparous women are generally not managed differently at this institution, and a standard value allowed for consistent comparisons.9 Because income, education, and insurance were not available through the obstetric electronic medical record, community health center care was used as a proxy for socioeconomic status. Low Apgar score was defined as a 5-minute score less than 7. Postpartum hemorrhage and shoulder dystocia were diagnosed clinically at the time of delivery by the primary physician or midwife.
Women with nonanomalous viable term singleton vertex gestations who reached full dilation were separated into two groups: those admitted in spontaneous labor and those admitted for induction of labor. Because prior research suggests that the effects of induction may vary by parity,14,15 separate analyses were planned for multiparous and nulliparous women. The duration of the second stage was compared between induced and spontaneous labor using both multivariable analysis and a time-to-event (Kaplan-Meier) analysis in which the event of interest was delivery. A Cochran Armitage trend test was used to detect trends in rates of complications per hour of the second stage in induced and spontaneous labor. Univariable and multivariable analyses were used to compare overall complication rates between induced and spontaneous labor. Covariates included in all multivariable analyses were: maternal age, body mass index, epidural use, birth weight, gestational age, midwifery care, year of delivery, and community health center care.
For continuous variables, Wilcoxon rank sum tests were used in unadjusted analyses and linear regression was used in adjusted analyses. For discrete variables, chi square tests were used in unadjusted analyses and logistic regression was used in adjusted analyses. SAS 9.2 was used for all analyses.
A total of 15,234 women met the inclusion criteria. Of these, six women were missing data on mode of delivery, 120 women were missing data on induction status, 291 were missing information on labor analgesia, and 90 were missing data on both induction status and labor analgesia. The remaining 14,727 women were included in the final analysis. Of these 14,727 women, 3,139 (21.3%) were induced and 11,588 (78.7%) were in spontaneous labor. Indications for induction were: postdates (46%), hypertension (18%), intrauterine growth restriction (11%), nonreassuring fetal testing (8%), diabetes (4%), oligohydramnios (3%), elective (3%), cholestasis (2%), other maternal disease (2%), and other (3%). These diagnoses were less common in the spontaneous labor group: postdates (9.4%), intrauterine growth restriction (1.3%), hypertension (1.5%), and diabetes (1.3%) (P<.01 for all diagnoses).
Table 1 compares the incidence of various risk factors for a prolonged second stage between women in induced and spontaneous labor. Women undergoing induction were more likely to be older, nulliparous, obese, more than 41 weeks of gestation, and under physician rather than midwifery care. They were also more likely to have regional anesthesia.
Among nulliparas, in unadjusted analyses, the length of the second stage of labor was slightly longer in induced labor (induction mean 102±78 minutes compared with spontaneous 107±83 minutes, P=.04). However, after adjusting for maternal age, body mass index, epidural use, birth weight, gestational age, midwifery care, year of delivery, and community health center care, there was no significant difference between the two groups (induction mean 102 minutes compared with spontaneous 103 minutes, P=.52). Among multiparas, there was no difference in duration of the second stage in adjusted or unadjusted analyses (induction mean 31±42 minutes compared with spontaneous 30±41 minutes, P=.99).
Figure 1 uses Kaplan-Meier analyses to compare time to delivery in induced compared with spontaneous labor among nulliparas (log-rank P=.002). After adjusting for potential confounders (maternal age, body mass index, epidural use, birth weight, gestational age, midwifery care, year of delivery, and community health center care), there was no significant difference between the groups overall (hazard ratio 1.01, 95% confidence interval [CI]0.96–1.07, P=.40) or after stratifying by mode of delivery (vaginal delivery hazard ratio 1.02, 95% CI 0.98–1.06, P=.43; cesarean delivery hazard ratio 0.91, 95% CI 0.76–1.09, P=.30).
As expected, the risk of complications increased as the duration of the second stage increased among women in spontaneous and induced labors (Table 2). Rates of all complications were significantly higher with longer second stage duration (Cochran-Armitage P<.05).
In multivariable analyses, after controlling for potential confounders, induction did not confer an increased risk of any labor and delivery complications among multiparas (Table 3). Among nulliparas, there was a higher odds of cesarean delivery (10.9% compared with 7.2%, adjusted odds ratio [OR] 1.32, 95% CI 1.01–1.71) in induced women compared with women in spontaneous labor. The most common indication for cesarean delivery in both the induced and spontaneous groups was active phase arrest (87.2% compared with 85.4%, P=.50). There was also a higher odds of postpartum hemorrhage (4.2% compared with 2.0%, OR 1.62, 95% CI 1.02–2.58) among induced nulliparous women compared with those in spontaneous labor. Induction did not confer an increased risk of other labor and delivery complications in nulliparas.
Because our goal was to isolate the independent effect (if any) of induction on the second stage of labor, we adjusted for birth weight in this analysis. Because some may argue that birth weight is not known at the time of delivery, we repeated our analyses without adjusting for birth weight, which did not change our findings.
We found that among women who reach full dilation, labor proceeds similarly regardless of induction status. Induction did not lengthen the duration of the second stage or increase the risk of prolonged second stage. At the observed 15% incidence of a prolonged second stage among nulliparas, our sample size allowed for more than 80% power to detect a 2% increase in the rate of a prolonged second stage with induction of labor in this population. Similar to previous studies, we found that in both induced and spontaneous labor, rates of complications increase with increasing duration of the second stage. Multiparas undergoing induction did not have an increased risk of complications compared with spontaneous labor. Among nulliparas, our data suggest an increased odds of cesarean delivery and postpartum hemorrhage in induction compared with spontaneous labor.
The appropriate duration of the second stage has not been defined.16 The goal of second-stage management is to maximize the chances of a vaginal delivery while minimizing risks of maternal and fetal complications. Recent studies suggest that increasing duration of the second stage is associated with maternal, but not fetal, complications.8,10 Our study supports this conclusion.
In deciding on management in the second stage, it is important to understand how modern obstetric interventions such as induction might affect duration and associated complications of the second stage. Although none of them focused specifically on the second stage, prior studies of induction have shown similar findings. Macer et al17 performed a retrospective case–control study of elective induction compared with spontaneous labor and noted no significant difference in length of the second stage between induced and spontaneous labor. In retrospective cohort studies, Vahratian15 noted no difference in the second stage among multiparas and Hoffman14 noted a very small difference in nulliparas. Notably, none of these studies included information on maternal or fetal complications of the second stage.
Some recent articles have suggested using women undergoing expectant management rather than spontaneous labor as the control group in studies of the optimal gestational age for induction, because this more accurately mirrors clinical decision-making.2 Our goal in this study was not to advise the physician deciding whether to induce a patient, but rather to inform patients and providers who manage the second stage of labor on the expected duration and complications of labor in the second stage. As a result, we elected to use women in spontaneous labor as the control group.
This study has limitations. First, our data are retrospective. Details of labor course such as timing of epidural, duration of pushing, and time of decision to deliver were not available. Although the obstetricians here were all part of one group sharing protocols and guidelines, there was no attempt to standardize management of the second stage. Length of the second stage was obtained from clinical information and does not represent the true biologic length of the second stage. However, our data do represent the time that each patient was managed as being in the second stage so may be clinically relevant. Retrospective data also limit our ability to adjust for confounders. Although we did adjust for diabetes, hypertension, and other diseases, we could not account for the difference in severity of those diseases in induced and spontaneous labors. We also could not adjust for unmeasured confounders. Both of these limitations may bias our results toward a worse outcome for induction. This may account for our finding of increased cesarean delivery and postpartum hemorrhage among induced nulliparas. However, these confounders are unlikely to account for our finding that induction does not increase the duration of the second stage or the risk of prolonged second stage, because these confounders would likely bias toward a worse outcome for induction. We were unable to include information about race or ethnicity into our model because we did not have adequate data about this variable. Our data on socioeconomic status were imprecise, because we used care at community health centers as a proxy. Finally, postpartum hemorrhage and chorioamnionitis were diagnosed by clinicians. It is possible that recording bias may have affected our results for these outcomes.
In conclusion, we find that once a woman reaches full dilation, induction does not change the expected duration of labor or the risk of prolonged second stage. Among induced multiparas who reach the second stage, there is no increased risk of maternal or fetal complications compared with spontaneous labor. Among nulliparas, induction may increase the risk of cesarean delivery and postpartum hemorrhage compared with spontaneous labor among those who reach the second stage. However, overall rates of these complications remain low. As rates of induction continue to rise, this information may be useful to physicians who counsel and manage patients in the second stage of labor.
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