Given the current cesarean section (C-section) rate, the prevalence of patients with a prior cesarean delivery is very high. Data processed from 2014 reveals a global average C-section rate of 18.6%, with particular countries having exceptionally high rates, such as Brazil (55.6%) and the Dominican Republic (56.4%).¹ The United States currently has a rate of 32.2%, well above the average.² Consequently, the number of patients who are potentially eligible for a TOLAC in a subsequent pregnancy is also increasing.

For patients requesting a TOLAC, the initial task is to assess if they are appropriate candidates for the procedure. Ideally, this evaluation would take place over the course of the patient’s prenatal care, involving information gathering and thorough discussions. Factors specific to the current pregnancy, the previous cesarean delivery (or deliveries), and the patient’s preferences and risk tolerance should all be considered. However, for any number of reasons, sometimes this evaluation has not been completed prior to the patient arriving at the hospital. Therefore it is critical that an obstetrics and gynecology (OB/GYN) hospitalist be able to perform an assessment of each patient’s situation, sometimes in rapid fashion, and formulate a plan that is appropriate for the patient, provider, and facility.

There are many patients for whom a TOLAC is appropriate. Women who have undergone one previous low-transverse cesarean delivery typically should be offered a TOLAC. Evaluations of this cohort reveal a rate of uterine rupture of < 1% (0.7%–0.9%).³ While many women fall into this category, there are other women for whom a TOLAC may be an option, but their cases may require additional consideration. Women with more than one previous low-transverse hysterotomy are one such group. Rates of uterine rupture for those with multiple C-sections are slightly increased over the baseline rate, as noted previously; however, the absolute risk itself remains low. Table 52-1 presents the various types of incisions and their implications with regards to the option of a TOLAC.

Studies evaluating the risk of uterine rupture after two previous presumed low-transverse cesarean deliveries show varying rates, with a meta-analysis calculating a pooled uterine rupture risk of 1.36% (0%–5.4%).⁴ Two studies specifically evaluating this particular population of patients revealed rates of uterine rupture of 0.9% and 1.8%.^5,6 As such, the American Congress of Obstetricians and Gynecologists (ACOG) and other national organizations, such as the Royal College of Obstetricians and Gynaecologists (RCOG), have agreed that it is reasonable for women with two prior low-transverse cesareans to be considered candidates for a TOLAC after evaluation of the patient’s overall pregnancy course.^7,8

In certain situations, the previous operative details may not be available, and consequently, the ability to verify if the patient’s previous cesarean delivery or deliveries were via low-transverse hysterotomy may not be possible. This may be particularly problematic when the patient’s prior operation was done in a facility where the operative record was recorded on paper. In this case, one must try to infer, based on the circumstances surrounding the prior delivery, if it was likely to have been a low-transverse hysterotomy. The patient may be able to provide information that can assist with this evaluation, such as that she went into labor at term with an incidental finding of breech presentation. If the patient delivered in another country and has ongoing contacts there, the medical professional may have the potential to obtain records via email or other digital media platforms.

Data on trial of labor after a previous cesarean with unknown scarring is somewhat limited. Previous studies have shown varying rates of uterine rupture (1%–5%), but they had relatively small sample sizes (only a couple of hundred subjects), and some of them included dehiscence noted at the time of scar palpation after vaginal delivery as part of this rate.^9–11 A recent analysis of the large data set from the Cesarean Registry project had 2460 patients with an unknown scar and a uterine rupture rate of 0.64%. This rate was not statistically significantly different from the rate for patients with a previous low-transverse scar. The authors mention that patients with an unknown scar in this analysis were more likely to be in spontaneous labor and to have had a history of a vaginal delivery or previous VBAC.¹² Thus it is appropriate that a patient with an unknown scar and (based on the history obtained) seems likely to have had a low-transverse incision may be offered the option of a TOLAC if no other variables would preclude this option.⁸ Table 52-2 lists a number of these studies.

Other factors may influence the decision to offer a patient a TOLAC; however, many of them have only limited information available about them. Such issues may include conditions related to the current pregnancy that put the uterus at risk of overdistension, such as a twin pregnancy or macrosomia. Others can be related to the previous cesarean, such as the surgical technique used and the duration for healing between deliveries.

With regard to the evaluation of the implications of a twin gestation on rates of uterine rupture and VBAC, there are limited studies, many of which have small sample sizes. Analysis of the twin gestations that were part of the Cesarean Registry initiated by the Maternal Fetal Medicine Unit had a total of 412 twin pregnancies, with at least 1 prior cesarean delivery for analysis; 186 of these cases attempted a TOLAC, with a success rate of 64.5%. The evaluation revealed a uterine rupture rate of 1.1% in the twin pregnancies undergoing a trial of labor, which was not statistically significant compared to the singleton TOLAC uterine rupture rate (0.7%) in that study’s sample. There was an increase in the amount of patients with endometritis; however, this was not significant when compared to twin pregnancies delivered by repeat cesarean.¹³ A secondary analysis of another large, retrospective VBAC cohort had similar findings. Their uterine rupture rate of the twin gestations was 1.1% (not statistically significant compared to 0.9% for singletons). The twin group was as likely as the singleton group to have a successful VBAC.¹⁴ A subsequent study with a larger cohort collected from billing data (1850 cases of TOLAC) also supports this estimate of the uterine rupture rate (0.9%).¹⁵ Thus it would seem reasonable that patients with a twin pregnancy, who are otherwise a candidate for a vaginal delivery and have a history of one previous low-transverse cesarean delivery, should be offered the option for a TOLAC.¹⁶

Macrosomia may put patients at risk for a number of adverse outcomes, including an increased risk of cesarean delivery, particularly due to an abnormal labor course, maternal hemorrhage, increased rates of third- and fourth-degree lacerations, and shoulder dystocia and its associated complications.¹⁷ In the setting of previous cesarean delivery, these issues may further complicate attempts at a TOLAC.

Evaluation of this specific factor is also limited by the fact that studies are retrospective and typically use birth weight instead of estimated fetal weight because the former value is readily available for collection as data. However, estimated fetal weight is the information that the clinician has on hand at the time of the decision to discuss pursuing a TOLAC with a patient.

A large case-control study (301 cases) did not reveal a statistically significant difference in the rate of uterine rupture for patients with newborns with birth weights over 4000 g. Although the success rate was lower for patients with birth weights > 4000 g (55% vs. 78%), there was still a reasonably successful vaginal delivery rate.¹⁸ Of note, further investigation in the setting of women whose only prior delivery was a cesarean (no history of previous vaginal deliveries) showed a slight increase in the rate of uterine rupture (1.6% vs. 1%), but did not reach statistical significance.¹⁹ This tendency was supported by Jastrow et al (2010) in a large retrospective data analysis which, after a stepwise regression, noted an odds ratio (OR) of 2.7 [95%; confidence interval (CI) 1.1–6.9] in those with a birth weight > 4000 g. More specifically, the uterine rupture rate for women with macrosomia (≥ 4000 g) and no previous vaginal delivery was 3.2% (6/188).²⁰ While macrosomia is not a contraindication to a TOLAC, the potential for a slightly increased rate of uterine rupture and slightly lower rate of success is worth considering, especially for patients who have not had a previous vaginal delivery. Figure 52-1 illustrates the elements that determine the success or failure of TOLAC.

There may also be other variables related to maternal health that may influence a provider’s decision to offer a patient a trial of labor due to its influence on the subsequent likelihood of success. These include maternal factors such as ethnicity, body mass index (BMI), and other medical problems. Most of these factors would be readily available to providers at the hospital. If the patient has received any prior medical or prenatal care, significant portions of their medical history would likely be available via medical records, or the patient may be able to communicate them. Maternal obesity has been associated with a decreased likelihood of VBAC and an increased risk of adverse outcomes. Decreased rates of successful VBAC have been noted in women with obesity (BMI > 30). Patients who were overweight (BMI > 25) did not have a statistically significant decrease compared to normal-BMI patients.²¹

A subset analysis of term patients in the MFMU Cesarean Registry revealed an increasing rate of failure with increasing BMI, with morbidly obese patients (BMI ≥ 40) experiencing a failure rate of 39.3%. Morbidly obese patients also had a higher rate of the combined metric of uterine rupture/dehiscence at 2.1% (p = 0.03).²² While a significantly elevated BMI itself is not a contraindication to a TOLAC, this factor should be considered in the overall evaluation of the patient’s situation.

Other maternal factors, such as age and ethnicity, have also been associated with the likelihood of a successful TOLAC. While Caucasian ethnicity has been found to have a higher likelihood of VBAC, Hispanic and African-American patients have a decreased probability of success.²³ The patient’s age also plays a role in the success rate, with older women (> 35 years old) experiencing a decreased rate of success.²⁴

Additional factors related to the previous cesarean delivery have been debated as well, including the number of layers in closure of the hysterotomy and the interval between deliveries. A previous study evaluating single- vs. double-layer closure (97.9% of suture used was chromic catgut) indicated the potential for increased rates of uterine rupture in the setting of single-layer closure, after adjustments, with an OR of 3.95 (95% CI: 1.35–11.49). The successful VBAC rate for this study was 76.3%.²⁵ However, a large cohort study of 7683 patients undergoing TOLAC in Sweden did not find any statistical significance in the rate of uterine rupture with regard to the number of layers of closure (OR 1.17; 95% CI 0.78–1.76) or with regards to the type of suture used.²⁶

The interpregnancy interval has been shown to be a significant factor in the risk of uterine rupture. Another study also evaluated the influence of interdelivery interval on the rate of uterine rupture. The researchers’ evaluation revealed an increased risk of rupture for a delivery interval of < 24 months, with an OR of 2.65 (95% CI 1.08–5.46). This was most pronounced in those women who had a single-layer closure (the uterine rupture rate for single-layer closure was 5.6% vs. 1.2% for double-layer).²⁷

This finding was further supported by a retrospective study analyzing pregnancy intervals in a large VBAC cohort. The overall rate of uterine rupture in the entire cohort was 0.9%. Separation by interpregnancy interval showed significant results; those patients with an interval < 6 months had an absolute risk of uterine rupture of 2.7%. In addition, this group had an increased risk for additional maternal morbidities (i.e. uterine rupture; bladder, ureter, or bowel injury; and uterine artery laceration).²⁸ Thus while a short pregnancy interval is not an absolute contraindication to a TOLAC, the risks must be weighed carefully given these findings. The indication for a previous cesarean has also been shown to influence the probability of a successful VBAC. A prior cesarean for cephalopelvic disproportion is associated with decreased likelihood of having a successful VBAC.²⁹

In some instances, patients with a history of a previous cesarean delivery may not be able to wait for spontaneous labor to occur. Medical disorders may require an induction of labor, or there may be logistical issues that warrant initiating the delivery process. An analysis of birth data by the Centers for Disease Control and Prevention (CDC) in 2014 revealed an induction of labor rate of 23.2%.³⁰ In the setting of induction of labor, the likelihood of a successful VBAC is decreased. Analysis of the MFMU Cesarean Registry (14,529 trials of labor at term) revealed the following VBAC success rates: 80.6% for spontaneous labor; 73.9% for oxytocin augmentation, and 67.4% for induction of labor.²⁹ Additional studies have supported this finding.^31,32

While a failed trial of labor has additional risks related to a C-section performed after attempted labor, perhaps the issue that receives the most attention is the risk of uterine rupture. Induction of labor, augmentation of labor, or both have been associated with an increase in the rate of uterine rupture. Zelop et al (1999) attempted to isolate the risk associated solely with induction itself by limiting their study population to patients with only one prior C-section. They found an increased risk of uterine rupture associated with induction of labor of 2.3% (13/560). This rate was significant compared to the rate for spontaneous labor, which was 0.7% (16/2214).³³ Of note, some patients in this study were given PGE₂ gel as an initial induction agent, followed by oxytocin.³³ More information is needed before a formal position on the use of the PGE₂ gel is established in the setting of TOLAC.¹⁶ Additional investigation surrounding other formulations such as PGE₂ may provide further insight into the specifics surrounding this particular agent, which is used in some countries, but not routinely.³¹

While augmentation is limited to oxytocin, induction may be initiated with a variety of different agents. Evaluation of the use of these agents in patients with a history of a previous C-section has revealed varying rates associated with uterine rupture. Prostaglandins historically have been associated with the most significant increase in the rate of uterine rupture compared to spontaneous labor or induction without prostaglandins.³⁴