Trial of labor after cesarean delivery (TOLAC) is a safe option for women with a history of cesarean delivery who desire a possible vaginal birth after cesarean delivery (VBAC). Offering TOLAC can help to reduce the population rate of cesarean delivery, as well as reduce each patient's individual risk of operative morbidity and future pregnancy complications. A National Institutes of Health consensus conference in 2010 recognized the safety of TOLAC and recommended increased access.1 – 3
Research has focused on the factors leading to successful TOLAC and the risks of undergoing a TOLAC attempt. Successful VBAC is more likely in women with a previous vaginal delivery, previous successful TOLAC, those with a nonrecurrent indication for cesarean, and those presenting for TOLAC in spontaneous labor.4 – 7 However, we know little about how labor progresses in women undergoing TOLAC. Clinicians can only optimize labor management of women undertaking TOLAC if normal labor parameters in this unique population are known. The data on labor progression for women with a previous cesarean delivery are sparse, but suggest that women with no previous vaginal deliveries labor similarly to nulliparous women, according to Friedman's criteria, and women with a previous vaginal delivery have labor curves similar to multiparous women.8 – 10
Our objective was to explore whether a uterine scar alters the progression of labor by using interval-censored regression to compare spontaneous labor curves in women undergoing TOLAC and those without a previous cesarean delivery.
MATERIALS AND METHODS
This was a planned secondary analysis of a retrospective cohort study conducted of all consecutive term vertex singletons from 2004 to 2008 at a single academic teaching hospital who underwent labor and achieved the second stage (10-cm dilation). This study was approved by the Washington University School of Medicine Human Research Protection Office.
Term gestation was defined as greater than or equal to 37 weeks and 0 days of gestation. Vertex presentation was confirmed by ultrasonography on admission. Diagnosis of spontaneous labor was determined by the admitting attending physician. To assess the natural course of labor in women with and without a uterine scar, parturient women were excluded if cervical ripening agents or oxytocin were used for induction or augmentation. An additional exclusion criterion was known fetal anomaly. Extensive data were collected from medical records, including obstetric and neonatal outcomes. All cervical examinations were extracted, including the time of the examination and the cervical dilation in centimeters.
Resident physicians provided the majority of labor and delivery services, including performing cervical examinations at regular intervals, usually every 2 hours. This institution has a culture of active management of labor, although no specific protocols were used for labor management, regardless of uterine scar. Eligibility criteria for TOLAC were consistent with the American College of Obstetricians and Gynecologists Practice Bulletin on VBAC that was current during enrollment.11 Absolute contraindications for TOLAC during that period included two or more cesarean deliveries, no history of a low transverse incision, and other standard contraindications for vaginal delivery, such as placenta previa. A recurrent indication for previous cesarean delivery, such as arrest of descent, may be taken into account while counseling patients on the likelihood of successful VBAC, but it is not an absolute contraindication to TOLAC.
Baseline characteristics of the study sample of spontaneously laboring women were tabulated, comparing women undergoing TOLAC to parturient women without history of a cesarean delivery. The χ2 test or Fisher exact test were used for categorical variables. Continuous variables were examined for normality using the Shapiro-Francia test and the Mann-Whitney U test was used to compare variables not normally distributed. Normally distributed continuous variables were compared using the Student t test.
A repeated-measures analysis with a ninth-degree polynomial model was used to construct average labor curves, stratified by TOLAC status and previous vaginal delivery. A repeated-measures analysis is required because cervical examinations in the same woman are correlated. Labor progression data are left-censored, because women present to be admitted for labor at varying cervical dilations. Thus, the time when labor begins and the progression of labor before women present to the hospital are unknown. The starting point was set as the time when dilation reached 10 cm, known for all participants, and time was calculated backwards. The curves were then reversed after construction to resemble a traditional labor curve, with time increasing from left to right.17 – 20
Interval-censored regression was used to estimate median duration of labor, centimeter by centimeter (traverse time), stratified by TOLAC status. Labor progression data are interval-censored, because cervical dilation is not continuously measured. Thus, one knows that the progression from 5 cm to 6 cm occurred at some time between two cervical examinations, but not exactly when. These time intervals were fitted to a log-normal distribution (right-skewed, accounting for some long labors, which create a long right tail of the distribution), and the median duration at each interval of dilation was estimated. Potentially confounding factors identified in univariable and stratified analyses were considered using backwards stepwise regression; only significant factors remained in the final models. The final models adjusted for race, maternal obesity, fetal macrosomia, and previous vaginal delivery. A subanalysis was performed with women without a previous vaginal delivery, comparing nulliparous women and women undergoing TOLAC without a previous vaginal delivery. All analyses were performed using Stata 10.0 and SAS 9.2.
Of 5,388 women in the sample, labor was induced in 1,647 (30.5%), and 1,720 (31.9%) were augmented using oxytocin. The rate of oxytocin augmentation in spontaneously laboring women undergoing TOLAC (45%) was similar to the rate in women without a history of cesarean delivery (46%; P=.09).
Only women in spontaneous labor without augmentation were included in this analysis (n=2,021), including 140 (7.0%) with a history of a cesarean delivery. Almost all women in the study sample delivered vaginally (99.0%). Of the 140 women undergoing TOLAC, 16.4% had their previous cesarean delivery for a recurrent indication. Only five women in the TOLAC group (3.6%) delivered by cesarean in the second stage, and none of these five had their previous cesarean for a recurrent indication. When the TOLAC and non-TOLAC groups were compared (Table 1), women undergoing TOLAC (n=140) were older (median age 26 compared with 23; P<.001) and had more previous pregnancies (median gravidity four compared with two; P<.001) on average. Women undergoing TOLAC had an increased rate of cesarean delivery (3.6% TOLAC compared with 0.7% non-TOLAC; Fisher exact P=.008). Most women (1,409; 70.0%) received epidural anesthesia; this did not differ significantly between the two study groups. Women attempting TOLAC did not significantly differ by other clinical variables, such as race, fetal macrosomia, or maternal obesity, to women laboring without a previous cesarean delivery.
Similar proportions in each group had a previous vaginal delivery (67.1% TOLAC compared with 68.3% non-TOLAC). Women without a history of vaginal delivery are described in Table 2. Women undergoing TOLAC without a previous vaginal delivery were older (median age 24 compared with 19; P<.001) than non-TOLAC laborers without a previous vaginal delivery (eg, nulliparous women). Laborers after cesarean delivery laborers in this group were less likely to be African-American (59% TOLAC compared with 75% non-TOLAC; Fisher exact P=.02).
There was no significant difference in rates of cervical dilation between the groups in both unadjusted and adjusted analyses (adjusted for race, maternal obesity, macrosomia, and previous vaginal delivery). The median time for dilation from 4 cm to 10 cm was 3.0 hours for TOLAC and 2.8 hours for non-TOLAC (P=.52; Table 3). When the analysis was performed only in women without a previous vaginal delivery, the median time of the active phase of labor did not differ based on presence of uterine scar (3.7 hours in both groups; P=.95; Table 4). Labor curves, stratified by TOLAC status, overlapped and showed a gradual transition from the latent to the active phase. There was no deceleration phase. Labor curves for women with a previous vaginal delivery are presented in Figure 1.
There was no significant difference in spontaneous labor progression curves or the rate of incremental cervical dilation between women undergoing TOLAC and those without a previous cesarean delivery. Additionally, women without a previous vaginal delivery, whether they were nulliparous or undergoing TOLAC, had similar patterns of labor progression. This population of term vertex singletons without augmentation or induction and who reached the second stage of labor represent women with normal labor curves with a high likelihood of achieving vaginal delivery. Only 0.7% of non-TOLAC and 4% of TOLAC women in this group were delivered by cesarean.
There is a paucity of clinically relevant data regarding labor management in TOLAC. Harlass et al9 conducted a retrospective analysis of 73 women undergoing TOLAC presenting in spontaneous labor matched to nulliparous and primiparous control group women. The authors determined the mean length of the first and second stage of labor, and women undergoing TOLAC had a similar length of labor as nulliparous women in the control group. Women whose previous cesarean was performed in the latent stage had a longer first stage of labor. However, the mean length of labor is of limited clinical value, because it only can be determined retrospectively and cannot guide intrapartum management.
Chazotte et al8 conducted a similar study using TOLAC parturient women matched to nulliparous and multiparous women in the control group. Stepwise discriminant analysis showed that women undergoing TOLAC had labor characteristics similar to those of nulliparous women if they had never had a vaginal delivery, and similar to those of multiparous women if they had. They found no between-group differences in the frequency of labor disorders (arrest, protraction, or prolonged latent phase).
In contrast to previous efforts, our study compares TOLAC and non-TOLAC labors using statistical techniques that take into account the unique characteristics of labor data, such as non-normal distribution and left-censoring. This analysis produces clinically useful information, such as the median time interval between two cervical dilations. Using these techniques, Zhang et al19,20 constructed labor curves in both historical and modern obstetric cohorts. In contrast to the traditional Friedman curves, Zhang describes a gradual transition from latent to active phase and lack of a deceleration phase. Our labor curves share these characteristics.
Our study is limited because of its retrospective nature in that we had no control over the frequency of cervical examinations. However, this represents the typical management of labor at our institution and can be generalized to similar institutions. In using a robust existing data set, our sample size was fixed. However, a post hoc sample size calculation with an alpha of 0.05 shows we had 90% power to detect a median difference in time to dilation from 4 cm to 10 cm of 0.4 hours. We propose that a difference in the length of the first stage of labor of 30 minutes would be clinically significant; there was sufficient power to detect such a clinically important difference.
Average labor curves represent a pattern of labor progression that can be used to diagnose and manage disorders of labor. Because the curves and incremental cervical dilation are not significantly different, women undergoing TOLAC should be held to the same standards as those without uterine scar. Uterine scar does not seem to affect the pattern of the first stage of spontaneous labor.
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