The second stage of labor begins when the cervix becomes fully dilated and ends with delivery of the neonate. Recent recommendations aimed at decreasing the rate of primary cesarean delivery have identified labor dystocia as a leading indication for abdominal delivery and have suggested that, if maternal and fetal status are satisfactory, allowing a longer second stage may help increase rates of vaginal delivery.1,2 A secondary analysis from a maternal-fetal medicine units randomized trial found that adverse neonatal outcomes were not significantly associated with the duration of the second stage.3 Similarly, in a trial of active compared with delayed pushing, a longer duration of active pushing was not associated with adverse neonatal outcomes, even in women who pushed for more than 3 hours.4 However, other studies have supported a longer second stage to be associated with increased risk of both maternal and neonatal adverse outcomes.5
In addition to being an important factor in avoiding a first cesarean delivery, management of the second stage may also be an important consideration for women undergoing labor after cesarean delivery. An important knowledge gap for management of labor after cesarean delivery is that risks and benefits for mothers and neonates of a prolonged second stage are unknown. Women undergoing labor after cesarean delivery are at increased risk for uterine rupture or dehiscence as well as other complications and may be less likely to achieve vaginal delivery than women without prior cesarean delivery.6 Knowledge of risks and chances of vaginal delivery based on the duration of the second stage during labor after cesarean delivery may be helpful for informed antepartum and intrapartum decision-making. The purpose of this study, therefore, was to assess the probability of vaginal delivery and risk of adverse maternal and neonatal outcomes as a function of second-stage duration in women with one prior cesarean delivery and no prior vaginal deliveries.
MATERIALS AND METHODS
This is an unplanned secondary analysis of the publically available Eunice Kennedy Shriver National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Cesarean Registry data set. This observational study was designed to assess clinical issues related to cesarean childbirth and recruited participants from 1999 through 2002 at 19 academic medical centers.6 The current analysis consists of women at 37 weeks of gestation or greater at delivery attempting a vaginal birth after cesarean delivery (VBAC) with one prior cesarean delivery who reached the second stage of labor with a documented, timed examination of full cervical dilatation. Only women with prior low transverse or unknown incision were included. Women with prior vaginal deliveries were excluded as were women delivering at less than 37 weeks of gestation. Women with more than one prior cesarean delivery were excluded. All pregnant women included in the analysis had a nonanomalous, live, singleton gestation. The duration of the second stage of labor was calculated as the time between full cervical documentation and delivery.
The primary outcome was mode of delivery, which included VBAC (both nonoperative and operative vaginal delivery) and repeat cesarean delivery. The analysis included demographics (age, race, and payer status), obstetric characteristics (induction or augmentation of labor, gestational age), and medical factors (body mass index [BMI, calculated as weight (kg)/[height (m)]2], medical conditions, drug and tobacco use). Other outcomes besides mode of delivery included length of second stage and adverse maternal and neonatal outcomes. Adverse maternal outcomes included chorioamnionitis, uterine atony, endometritis, red cell transfusion, hysterectomy, and uterine dehiscence, defined as disruption of the uterine muscle with intact serosa, or rupture, defined as disruption or tear of the uterine muscle and visceral peritoneum or separation of the uterine muscle with extension to the bladder or broad ligament.7 Adverse neonatal outcomes included cord pH acidemia at delivery (arterial cord pH less than 7.10), Apgar score less than 6 at 5 minutes, neonatal intensive care unit (NICU) admission, and ventilatory support in the first 24 hours of life. These outcomes were assessed individually and a neonatal morbidity composite consisting of acidemia, ventilator support, and NICU admission was also calculated.
Medical, obstetric, and demographics characteristics were evaluated for all patients and then dichotomously for patients with second stage less than 3 hours and 3 hours or greater. Probability of vaginal delivery based on individual medical, obstetric, and demographic characteristics was similarly evaluated for all patients and then restricted to patients with second stage 3 hours or greater. The proportion of deliveries that occurred by cesarean, by operative vaginal, and by spontaneous vaginal delivery was determined for second stages of length less than 1 hour, 1 hour or greater to less than 2 hours, 2 hours or greater to less than 3 hours, 3 hours or greater to 4 hours, and 4 hours or greater. Adjusted rate ratios for successful vaginal delivery with 95% CIs accounting for demographic, obstetric, and medical risk factors were derived from fitting log-linear regression models based on the Poisson distribution. Models were run for all patients in the cohort and for patients with second-stage length 3 hours or greater. Second stage was also dichotomized into less than 3 hours and 3 hours or greater and risk of individual adverse maternal (chorioamnionitis, uterine atony, endometritis, and transfusion) and neonatal outcomes (fetal acidemia, Apgar score less than 6 at 5 minutes, NICU admission, ventilator support, and morbidity composite) were compared separately for vaginal and cesarean delivery using the χ2 test. Because uterine dehiscence or rupture may be a cause of cesarean delivery, this outcome was compared based on second-stage length for all deliveries. A second-stage duration of 3 hours or greater has been used in prior studies for defining prolonged second stage.3
We undertook two sensitivity analyses. First, because epidural anesthesia may be associated with longer second-stage duration, we evaluated second-stage length and success of vaginal delivery based on whether or not women received epidural anesthesia (including both epidural and spinal–epidural anesthesia). Second, because success of vaginal delivery may be associated with the indication for prior cesarean delivery, we dichotomized the cohort into women who had undergone an initial cesarean delivery for recurring or nonrecurring indications. Recurring indications included cephalopelvic disproportion, failure to progress, failed induction, and failed operative vaginal delivery. Nonrecurring indications included nonreassuring fetal tracing, malpresentation, multiple gestation, placenta previa, and other diagnoses. Women with an unknown indication for prior cesarean delivery were excluded from this analysis. Probability of successful vaginal delivery by second-stage duration based on whether the prior cesarean delivery was for a recurring on nonrecurring indication was evaluated. All statistical analyses were performed using SAS 9.4. Our study was found to be exempt from institutional review board approval, because there is no requirement for projects that do not meet the federal definitions of research with human subjects to be submitted to the institutional review board.
A total of 4,579 women met inclusion criteria and were included in the study (Fig. 1). Of the included women, 4,147 (90.6%) had a successful VBAC. Of the 432 women undergoing cesarean delivery, the most common indications were cephalopelvic disproportion (n=187), failure to progress (n=143), and nonreassuring fetal monitoring (n=71) (Table 1). The rate of successful VBAC decreased as second-stage length increased (Fig. 2) with 97.3% of women who delivered in less than 1 hour having a spontaneous or operative vaginal delivery (2,536/2,607 women). As second stage increased, successful vaginal delivery rates decreased: 97.3% at less than 1 hour (95% CI 96.6–97.9%), 91.5% at 1 to less than 2 hours (95% CI 89.8–93.1%), 78.5% at 2 to less than 3 hours (95% CI 74.5–82.1%), 62.3% at 3 to less than 4 hours (95% CI 55.2–69.1%), and 45.6% at 4 hours or greater (95% CI 37.7–53.7%) (P<.01). Operative vaginal and cesarean deliveries were also significantly more likely as second-stage duration increased (P<.01). Operative vaginal delivery occurred in 14.5% of vaginal deliveries with second stage less than 1 hour, 18.2% of vaginal deliveries occurring between 1 and less than 2 hours, 32.8% of vaginal deliveries occurring between 2 and less than 3 hours, 49.2% of vaginal deliveries occurring from 3 to less than 4 hours, and 56.2% of all deliveries occurring at 4 hours or greater (P<.01 for differential rates of operative delivery across second-stage duration; Table 2).
Women undelivered at 3 hours were significantly more likely to be of advanced maternal age, to have private insurance, to be white, to have required labor augmentation, and to be overweight or obese than women delivered with a second stage of less than 3 hours (Table 3). In the adjusted models for vaginal delivery for both the entire cohort and women undelivered at 3 hours of second-stage duration, no factors were associated with a statistically significantly increased or decreased likelihood of vaginal delivery (Table 4) with the exception of maternal age 35 years or older with maternal age 20 to younger than 25 years as a referent (adjusted rate ratio 0.89 95% CI 0.79–0.99). Factors included in the models included prepregnancy BMI, maternal age, race, payer, induction status, gestational age, smoking, drug use, and medical conditions.
For all adverse maternal outcomes (chorioamnionitis, uterine atony, endometritis, red cell transfusion, and uterine rupture or dehiscence), risk increased as second-stage duration increased (Fig. 3A) with the exception of hysterectomy of which there was only one case. These differences were statistically significant except for uterine atony (P=.08). Specifically, risk of uterine rupture or dehiscence increased with deliveries with second-stage length less than 1 hour, 1 to less than 2 hours, 2 to less than 3 hours, and 3 hours or greater (0.7% vs 1.4% vs 1.5% vs 3.1%, P<.001; Table 5). Although the likelihood of uterine rupture or dehiscence was higher in women with second stage 3 hours or greater compared with less than 3 hours (3.1% [11/359] vs 1.0% [41/4,220], P=.01), because more women delivered with second stage less than 3 hours, the absolute number of these events was higher in the less than 3-hour cohort.
In comparison, risk of each of the adverse neonatal outcomes (fetal acidemia, low Apgar scores, NICU admission, and ventilatory support) was not significantly associated with longer second-stage duration. Similarly, there was no additional risk of a composite of outcomes consisting of academia, NICU admission, and ventilatory support when second-stage duration of less than and greater than 3 hours were compared (Fig. 3B). Second-stage durations exceeding 3 hours were not significantly associated with increased risk of any of the maternal or neonatal adverse outcomes by mode of delivery (Table 6).
Results from the sensitivity analyses evaluating women 1) with (n=3,825) and without (n=754) epidural anesthesia and 2) with recurring (n=1,771) compared with nonrecurring indications (n=2,628) for cesarean delivery are shown in Table 2. Women with epidural anesthesia were more likely to deliver with second stage less than 1 hour than those without neuraxial anesthesia (52.7% vs 78.6%, P<.01) and less likely to have second stage 3 hours or greater (2.5% vs 8.9%, P<.01). More than 90% of women with epidural anesthesia delivered vaginally compared with 92% of women without neuraxial anesthesia (P=.07). Similarly, women with nonrecurring compared with recurring indications for cesarean delivery were more likely to deliver with second stage less than 1 hour (58.5% vs 49.1%, P<.01), more likely to achieve vaginal delivery (93.9% vs 85.3%, P<.01), and less likely to have second stage 3 hours or greater (6.2% vs 10.6%, P<.01).
Data from this secondary analysis demonstrated that a significant proportion of women with longer second stage undergoing labor after cesarean delivery successfully achieved vaginal delivery, although probability of success decreased as second stage increased. Prolonged second stage was associated with increased risk of maternal morbidity; this added risk appeared to be attributable cesarean delivery becoming more likely as second-stage duration increased rather than vaginal or cesarean delivery individually becoming riskier. When vaginal and cesarean deliveries were analyzed separately based on whether the second stage was 3 hours or greater, no significant differences in risk of adverse maternal and neonatal outcomes were noted. Because maternal risk appeared associated with mode of delivery rather than second-stage length, it is unclear whether performing cesarean delivery sooner could have averted adverse maternal outcomes. Recommendations from the American College of Obstetricians and Gynecologists aimed at decreasing the rate of primary cesarean delivery support allowing a longer second stage to decrease the diagnosis of second-stage arrest and increase the likelihood of vaginal delivery.2 Although second-stage outcomes have been investigated in nulliparous cohorts,3,4 data on second stage during labor after cesarean delivery are limited. These data demonstrate similar findings to those in nulliparous women.3
In interpreting the findings of this study, there are several important considerations. First, this study is limited in determining the causal relationship between uterine rupture or dehiscence and second-stage length. When uterine rupture or dehiscence is diagnosed during cesarean delivery for arrest of descent, it is not possible to determine whether the event occurred before or during the second stage; rupture or dehiscence could occur before the second stage and only become “unmasked” clinically by concerning heart rate patterns or maternal findings apparent with the frequent uterine contractions and pushing that accompany the second stage. For this reason, it is not possible to know whether the longer second-stage duration was the result of undiagnosed uterine rupture or dehiscence or the cause. Because of these limitations, this analysis is not able to determine whether performing cesarean delivery earlier in the second stage would significantly reduce the risk of uterine rupture or dehiscence; indeed, more than three fourths of uterine rupture or dehiscence cases were diagnosed at deliveries with second stage less than 3 hours. An appropriate clinical takeaway from these findings is that patients who remain undelivered after a second stage of 3 hours are at increased risk of uterine rupture and should be monitored vigilantly.
A second important consideration in interpreting this analysis is that the centers where the study took place are not necessarily representative of obstetric care provided nationally. A commonly encountered limitation of randomized controlled trials and large prospective observational research trials is that they are often performed in academic or large community hospitals with facilities that differ from other smaller community centers significantly in terms of personnel, patients, resources, patient populations, and practice patterns. Maternal and neonatal risk related to labor after cesarean delivery may differ significantly depending on staffing, anesthesia, operating room availability, and other factors that affect prompt identification of emergent complications such as uterine rupture and expedient delivery. The patient population in this analysis had a particularly high rate of successful vaginal delivery and may not be representative of the entire obstetric population. A third important consideration in interpreting this study is that data collection began almost 20 years ago. Numerous changes in the obstetric population have occurred including increased rates of obesity and advanced maternal age.8–11 Although an important finding from this analysis was that we could not identify any factors associated with large changes in likelihood of vaginal delivery once second stage was achieved, an urgent goal for future research will be to validate these findings in contemporary populations. A fourth important consideration is that this study utilized data that were not collected specifically to evaluate outcomes relative to second-stage length. Although secondary analyses of larger trials represent a cost-effective and expedient means of analyzing outcomes, data collection methods tailored to a specific research question—in this case more frequent and protocol-driven examinations to accurately determine time of full cervical dilation—are often not available. A fifth consideration is that we did not have time of pushing relative to second-stage length and that the relationships seen may be dependent more on prolonged length of pushing than overall second-stage length. Sixth, we note that given the relatively small number of adverse outcomes evaluated, we were only able to perform unadjusted analyses; multivariable models with larger populations may be better able to characterize the relationship between the exposure (second-stage length) and adverse outcomes specifically evaluating the contribution of risk from mode of delivery.12 Likewise, for some comparisons, such as subgroup comparisons based on mode of delivery or for neonatal outcomes, our analysis may not have powered to detect smaller differentials based on second-stage length.
We found that even with prolonged second stage, a clinically significant proportion of women successfully achieved vaginal delivery. Given that risk of adverse maternal outcomes, including uterine rupture or dehiscence, is more common as the second stage lengthens, women undergoing labor after cesarean delivery with prolonged second stage should undergo close observation of fetal heart rate monitoring, maternal vital signs, and symptoms suggestive of uterine rupture or dehiscence. Although neonatal outcomes were similar based on second-stage length, an important limitation of this study is that it was performed at larger academic centers and care should taken when extrapolating these findings to other hospital settings.
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© 2018 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.
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