There are few high-quality data available to guide selection of trial of labor candidates (Guise, 2004; Hashima, 2004). That said, having fewer complicating risk factors increases the likelihood of success (Gregory, 2008). Several algorithms and nomograms have been developed to aid prediction, but none has demonstrated reasonable prognostic value (Grobman, 2007b, 2008, 2009; Macones, 2006; Metz, 2013; Srinivas, 2007). Use of a predictive model for failed trial of labor, however, was found to be somewhat predictive of uterine rupture or dehiscence (Stanhope, 2013). Despite these limitations for precision, several points are pertinent to the evaluation of these women and are described in the next sections. Current recommendations of the American College of Obstetricians and Gynecologists (2013a) are that most women with one previous cesarean delivery with a low-transverse hysterotomy are candidates, and if appropriate, they should be counseled regarding a trial of labor versus elective repeat cesarean delivery.
The type and number of prior cesarean deliveries are overriding factors in recommending a trial of labor. Women with one prior low-transverse hysterotomy have the lowest risk of symptomatic scar separation (Table 31-3). The highest risks are with prior vertical incisions extending into the fundus, such as the classical incision shown in Figure 31-2 (Society for Maternal Fetal Medicine, 2013). Importantly, in some women, the classical scar will rupture before the onset of labor, and this can happen several weeks before term. In a review of 157 women with a prior classical cesarean delivery, one woman had a complete uterine rupture before the onset of labor, whereas 9 percent had a uterine dehiscence (Chauhan, 2002).
Ruptured vertical cesarean section scar (arrow) identified at time of repeat cesarean delivery early in labor. The two black asterisks to the left indicate some sites of densely adhered omentum.
TABLE 31-3Types of Prior Uterine Incisions and Estimated Risks for Uterine Rupture ||Download (.pdf) TABLE 31-3 Types of Prior Uterine Incisions and Estimated Risks for Uterine Rupture
|Prior Incision ||Estimated Rupture Rate (%) |
|Classical ||2–9 |
|T-shaped ||4–9 |
|Low-verticala ||1–7 |
|One low-transverse ||0.2–0.9 |
|Multiple low-transverse ||0.9–1.8 |
|Prior preterm cesarean delivery ||“increased” |
|Prior uterine rupture || |
| Lower segment ||2–6 |
| Upper uterus ||9–32 |
The risk of uterine rupture in women with a prior vertical incision that did not extend into the fundus is unclear. Martin (1997) and Shipp (1999) and their coworkers reported that these low-vertical uterine incisions did not have an increased risk for rupture compared with low-transverse incisions. The American College of Obstetricians and Gynecologists (2013a) concluded that although there is limited evidence, women with a prior vertical incision in the lower uterine segment without fundal extension may be candidates for a trial of labor. This is in contrast to prior classical or T-shaped uterine incisions, which are considered by most as contraindications to labor.
Currently, there are few indications for a primary vertical incision (Osmundson, 2013). In those instances—for example, preterm breech fetus with an undeveloped lower segment—the “low vertical” incision almost invariably extends into the active segment. It is not known, however, how far upward the incision has to extend before risks become those of a true classical incision. It is helpful in the operative report to document its exact extent.
Prior preterm cesarean delivery may have an increased risk for rupture. Although the type of prior incision was not known, Sciscione and associates (2008) reported that women who had a prior preterm cesarean delivery were twice as likely to suffer a uterine rupture compared with those with a prior term cesarean delivery. This may be in part explained by the increased likelihood with a preterm fetus of extension of the uterine incision into the contractile portion. Conversely, Harper and colleagues (2009) did not find a significantly increased rate of rupture during subsequent VBAC in women with a prior cesarean delivery performed at or before 34 weeks.
There are special considerations for women with uterine malformations who have undergone cesarean delivery. Earlier reports suggested that the uterine rupture risk in a subsequent pregnancy was increased compared with the risk in those with a prior low-transverse hysterotomy and normally formed uterus (Ravasia, 1999). But, in a study of 103 women with müllerian duct anomalies who had one prior cesarean delivery and who attempted a trial of labor, there were no cases of uterine rupture (Erez, 2007). Given the wide range of risk for uterine rupture associated with the various uterine incision types, it is not surprising that most fellows of the American College of Obstetricians and Gynecologists consider the type of prior incision to be the most important factor when considering a trial of labor (Coleman, 2005).
As discussed in Chapter 30 (Placental Delivery), the low-transverse hysterotomy incision can be sutured in either one or two layers. Whether the risk of subsequent uterine rupture is affected by these is unclear. Chapman (1997) and Tucker (1993) and their associates found no relationship between a one- and two-layer closure and the risk of subsequent uterine rupture. And although Durnwald and Mercer (2003) also found no increased risk of rupture, they reported that uterine dehiscence was more common after single-layer closure. In contrast, Bujold and coworkers (2002) found that a single-layer closure was associated with nearly a fourfold increased risk of rupture compared with a double-layer closure. In response, Vidaeff and Lucas (2003) argued that experimental models have not demonstrated advantages of a double-layer closure and that the evidence is insufficient to routinely recommend a double-layer closure. At Parkland Hospital we routinely suture the low-segment incision with one running locking suture.
Number of Prior Cesarean Incisions
There are at least three studies that reported a doubling of the rupture rate—from approximately 0.6 to 1.85 percent—in women with two versus one prior transverse hysterotomy incision (Macones, 2005a; Miller, 1994; Tahseen, 2010). In contrast, analysis of the MFMU Network database by Landon and colleagues (2006) did not confirm this. Instead, they reported an insignificant difference in the uterine rupture rate in 975 women with multiple prior cesarean deliveries compared with 16,915 women with a single prior operation—0.9 versus 0.7 percent, respectively. As discussed subsequently, other serious maternal morbidity increases along with the number of prior cesarean deliveries (Marshall, 2011).
Imaging of Prior Incision
Sonographic measurement of a prior hysterotomy incision has been used to predict the likelihood of rupture with a trial of labor. Large defects in the hysterotomy scar in a nonpregnant uterus forecast a greater risk for rupture (Osser, 2011). Naji and associates (2013a,b) found that the residual myometrial thickness decreased as pregnancy progressed and that rupture correlates with a thinner scar. From their review, however, Jastrow and coworkers (2010) found no ideal thickness to be suitably predictive. The optimal safe threshold value for the lower uterine segment—smallest measurement from amnionic fluid to bladder—ranged from 2.0 to 3.5 mm. For the myometrial layer—smallest measurement of the hypoechoic portion of the lower segment, it was 1.4 to 2.0 mm. With a lower segment < 2.0 mm, the risk of uterine rupture was increased 11-fold. With myometrial thickness < 1.4 mm, uterine rupture was increased fivefold. The measurements, however, are not predictive for an individual woman (Bergeron, 2009).
Women who have previously sustained a uterine rupture are at increased risk for recurrence during a subsequent attempted VBAC. As shown in Table 31-3, those with a previous low-segment rupture have a 2- to 6-percent recurrence risk, whereas prior upper segment uterine rupture confers a 9- to 32-percent risk (Reyes-Ceja, 1969; Ritchie, 1971). We believe that women with a prior uterine rupture or classical or T-shaped incision ideally should undergo repeat cesarean delivery when fetal pulmonary maturity is assured, and preferably before the onset of labor. They should also be counseled regarding the hazards of unattended labor and signs of possible uterine rupture.
Magnetic resonance imaging studies of myometrial healing suggest that complete uterine involution and restoration of anatomy may require at least 6 months (Dicle, 1997). To explore this further, Shipp and colleagues (2001) examined the relationship between interdelivery interval and uterine rupture in 2409 women who had one prior cesarean delivery. There were 29 women with a uterine rupture—1.4 percent. Interdelivery intervals of ≤ 18 months were associated with a threefold increased risk of symptomatic rupture during a subsequent trial of labor compared with intervals > 18 months. Similarly, Stamilio and coworkers (2007) noted a threefold increased risk of uterine rupture in women with an interpregnancy interval of < 6 months compared with one ≥ 6 months. But they also reported that interpregnancy intervals of 6 to 18 months did not significantly increase the risk.
Prior vaginal delivery, either before or after a cesarean birth, significantly improves the prognosis for a subsequent vaginal delivery with either spontaneous or induced labor (Grinstead, 2004; Hendler, 2004; Mercer, 2008). Prior vaginal delivery also lowers the risk of subsequent uterine rupture and other morbidities (Cahill, 2006; Hochler, 2014; Zelop, 2000). Indeed, the most favorable prognostic factor is prior vaginal delivery.
Indication for Prior Cesarean Delivery
Considering all women who elect trial of labor, 60 to 80 percent will have a successful vaginal delivery (American College of Obstetricians and Gynecologists, 2013a). Women with a nonrecurring indication—for example, breech presentation—have the highest success rate of nearly 90 percent (Wing, 1999). Those with a prior cesarean delivery for fetal compromise have an approximately 80-percent success rate, and for those done for labor arrest, success rates approximate 60 percent (Bujold, 2001; Peaceman, 2006). Prior second-stage cesarean delivery can be associated with second-stage uterine rupture in a subsequent pregnancy (Jastrow, 2013).
Most studies show that increasing fetal size is inversely related to successful trial of labor. The risk for uterine rupture is less robust. Zelop and associates (2001) compared the outcomes of almost 2750 women undergoing a trial of labor, and 1.1 percent had a uterine rupture. The rate increased—albeit not significantly—with increasing fetal weight. The rate was 1.0 percent for fetal weight < 4000 g, 1.6 percent for > 4000 g, and 2.4 percent for > 4250 g. Similarly, Elkousy and colleagues (2003) reported that the relative risk of rupture doubled if birthweight was > 4000 g. Conversely, Baron and coworkers (2013a) did not find increased uterine rupture with birthweights > 4000 g.
Fetal size at the opposite end of the spectrum may increase the chances of a successful VBAC. Specifically, women who attempt a trial of labor with a preterm fetus have higher successful VBAC rates and lower rupture rates (Durnwald, 2006; Quiñones, 2005).
Perhaps surprisingly, twin pregnancy does not appear to increase the uterine rupture risk with trial of labor. Ford and associates (2006) analyzed the outcomes of 1850 such women and reported a 45-percent successful vaginal delivery rate and a rupture rate of 0.9 percent. Similar studies by Cahill (2005) and Varner (2007) and their colleagues reported rupture rates of 0.7 to 1.1 percent and vaginal delivery rates of 75 to 85 percent. According to the American College of Obstetricians and Gynecologists (2013a), women with twins and a prior low-transverse hysterotomy who are otherwise candidates for vaginal delivery can safely undergo a trial of labor.
Obesity decreases the success rate of trial of labor. Hibbard and coworkers (2006) reported the following vaginal delivery rates: 85 percent with a normal body mass index (BMI), 78 percent with a BMI between 25 and 30, 70 percent with a BMI between 30 and 40, and 61 percent with a BMI of 40 or greater. Similar findings were reported by Juhasz and associates (2005).
Most women with a prior cesarean delivery and fetal death in the current pregnancy would prefer a VBAC. Although fetal concerns are obviated, available data suggest that maternal risks are increased. Nearly 46,000 women with a prior cesarean delivery in the Network database had a total of 209 fetal deaths (Ramirez, 2010). There were 76 percent who had a trial of labor with a success rate of 87 percent. Overall, the rupture rate was 2.4 percent. Four of five ruptures were during induction in 116 women with one prior transverse incision—3.4 percent.