Materials and Methods

This was a retrospective cohort study from 2004-2008 of all consecutive women who were admitted for delivery with singleton gestations who reached the second stage of labor (10-cm dilation) and delivered vaginally. Women who were eligible for this cohort had a vertex-presenting fetus that had been confirmed by ultrasound on presentation to labor and delivery. Women included in this study had a history of cesarean delivery. All pregnancies with known fetal anomalies were excluded from this study. This study occurred at a single, academic teaching hospital and was approved by the Washington University School of Medicine Human Research Protection Office.

Criteria for labor after cesarean eligibility were consistent with the American College of Obstetricians and Gynecologists Practice Bulletin on VBAC that was current during the enrollment period. Women were considered ineligible if they had an absolute contraindication for VBAC such as a history of vertical or low vertical hysterotomy. Women who had an absolute contraindication for vaginal delivery, such as a placenta previa, were also considered ineligible for this study. Women who had an unknown scar were considered candidates for VBAC if they did not have any risk factors for a vertical hysterotomy such as preterm birth at the approval of the attending physician. An indication for previous cesarean delivery that may recur (such as arrest of descent, arrest of dilation, or failed induction of labor) was not considered a contraindication to VBAC. However, this information was often used when patients were counseled before proceeding with labor after cesarean delivery.

Trained research personnel extracted detailed maternal sociodemographic information, obstetric and medical history, intrapartum course, and neonatal outcomes from the medical record. The admitting attending physician determined the diagnosis of spontaneous labor. Cervical examination data that were extracted included the time of the examination and the cervical dilation in centimeters. Patients typically were examined every 2 hours; resident physicians were the primary providers to perform the cervical examinations. Our institution has a culture of active management of labor, although specific protocols for the management of labor were at the discretion of the managing attending physician. Induction was performed with oxytocin or oxytocin and a Foley balloon catheter. Our institutional policy for oxytocin administration for women who undergo labor after cesarean delivery is to start the oxytocin at 2 mU/min and to increase by 2 mU/min every 20 minutes until the uterine contraction pattern is adequate or until a maximum of 20 mU/min.

Women who underwent induction of labor after cesarean delivery were compared with those with spontaneous labor. Baseline characteristics were examined. Categoric variables were examined with the use of the χ ² test or Fisher exact test; continuous variables were examined for normality with the use of the Shapiro-Francia test. The Student t test was used to compare normally distributed continuous variables; the Mann-Whitney U test was used for variables not normally distributed.

Labor curves were constructed for women who underwent an induction and were compared with those women with spontaneous labor. Interval-censored regression was used to estimate the median duration of labor, centimeter by centimeter, stratified by induction status. The specific time of each advancement in cervical dilation is not precisely known, because cervical dilation is not measured constantly; therefore, only the times at which a cervical examination occurred were known. These time intervals (from 1 cervical examination to the next cervical examination) were fitted to a log-normal distribution, and the median duration at each interval of dilation was estimated.

A repeated-measures analysis with a ninth-degree polynomial model was used to create average labor curves. These were stratified by labor induction status (induction vs spontaneous). The starting point for the labor curve construction was set at 10 cm, because all patients in this study reached the second stage. This backward construction was performed because women arrive at the hospital typically after cervical dilation has already occurred, and the exact timing of the progression before arriving at the hospital is unknown. The curves were then reversed, with time increasing from left-to-right along the x-axis according to the typical representation of time progression. Potentially confounding factors that were identified in univariate and stratified analyses were considered with the use of backwards-stepwise regression; only significant factors remained in the final models. The final models were adjusted for maternal obesity, previous vaginal delivery, use of regional anesthesia, and fetal macrosomia (defined as birthweight >4000 g). All analyses were performed using Stata software (version 10.0; StataCorp, College Station, TX) and SAS software (version 9.2; SAS Institute Inc, Cary, NC).

Results

Of the 10,564 women in our birth cohort, there were 473 women included in this study who achieved VBAC and whose labor type could be ascertained. Of these, 234 women (49%) underwent an induction of labor. Baseline characteristics are featured in Table 1 . Women who were induced were more likely to deliver at a greater gestational age and be of lower gravidity. They were less likely to have a previous vaginal delivery. They were more likely to be obese, have an epidural, and deliver a macrosomic infant. As expected, the induction group was less likely to have a Bishop score ≥5 at admission. There were no differences in maternal age, rate of African American race, or rate of spontaneous vaginal delivery. There was also no difference between the rates of hypertension in pregnancy or pregestational or gestational diabetes mellitus. Approximately 12% of the induction group and 9% of the spontaneous group had a “recurring indication” for previous cesarean delivery. There were no differences between the 2 groups in the rates of previous indication for cesarean delivery documented by the provider as failure to progress, failed induction, cephalopelvic disproportion, or arrest of labor. Women in the induction group had a significantly longer second stage of labor compared with the spontaneous group.

The indications for induction for labor are listed in Table 2 . Information was only available for 106 of the 234 women who were undergoing induction, and women could have >1 indication for induction. For women with a known indication for induction, the most common reasons were elective, premature rupture of membranes, preeclampsia, and oligohydramnios.

Women who underwent an induction of labor had a longer total time in labor than those in the spontaneous labor group: 5.1 hours (95% confidence interval [CI], 1.4–18.2) compared with 3.1 hours (95% CI, 0.9–11.0; P < .01; Table 3 ). Women in the induction group took longer to advance 1 cm in dilation at 3-7 cm. After 7 cm, the time to progress 1 cm in dilation was not different from 7-8 cm and 9-10 cm.

Table 3

Time to reach a given cervical dilation

Cervical dilation, cm	Median hours (95% confidence interval)		P value
	Induced (n = 234)	Spontaneous (n = 239)
4-10	5.1 (1.4–18.2)	3.1 (0.9–11.0)	< .01
7-10	0.9 (0.1–7.0)	0.6 (0.1–4.7)	.01
3-4	1.1 (0.2–7.4)	0.4 (0.1–2.9)	.02
4-5	1.1 (0.2–7.0)	0.6 (0.1–3.8)	.02
5-6	0.6 (0.1–5.0)	0.3 (0.04–2.6)	.01
6-7	0.5 (0.1–3.3)	0.2 (0.03–1.3)	< .01
7-8	0.2 (0.03–1.8)	0.2 (0.03–1.4)	.29
8-9	0.2 (0.03–1.5)	0.1 (0.02–0.8)	< .01
9-10	0.2 (0.02–1.2)	0.2 (0.02–1.1)	.50

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