When comparing birth weights in the 2 groups (Table 1), those patients with a successful VBAC had a significantly lower average birth weight (3,353.44 ± 553.76 g) than those who failed VBAC (3,523.10 ± 470.31 g; P < .001). Surprisingly, the patients with successful VBAC attempts had significantly smaller babies in the previous pregnancy (3,238.83 ± 641.67 g) than in the current pregnancy (3,353.44 ± 553.76 g; P < .001), and similarly, the babies of those patients who failed VBAC in the current pregnancy were also significantly smaller in the previous pregnancy (3,434.31 ± 553.76 g versus 3,523.10 ± 470.31 g; P = .046).
A large portion (42.7%) of the total number of patients attempting VBAC were induced, and of these, 72.8% went on to deliver vaginally. Of the remaining 697 patients who were not induced, 80.3% delivered vaginally (P = .002). However, when we controlled for diabetes, birth weight, and recurrent indications, induction no longer significantly affected VBAC outcome (OR 0.83, 95% CI 0.91–2.71) (Table 3).
The overall VBAC success rate of gestational and pregestational diabetics was 60.7%, compared with 78.8% in patients without diabetes (P < .001). When neonatal birth weight was analyzed separately, those patients whose infants weighed 4,000 g or more had a 67.5% VBAC success, whereas those weighing under 4,000 g had a 78.1% success rate (P = .01). The VBAC success rate was 69.1% when there was a recurrent indication for cesarean delivery, but rose to 85.0% if the indication was nonrecurring (P < .001). The presence of diabetes (OR 0.42, 95% CI 0.28–0.62), birth weight of 4,000 g or more (OR 0.58, 95% CI 0.38–0.88), and a recurrent indication for cesarean delivery (OR 0.39, 95% CI 0.30–0.52) each significantly decreased VBAC success and were independently associated with poor outcome (P < .001).
The overall uterine rupture rate was 1.56%. There was no significant difference in the rate of uterine rupture in the previous VBAC group compared with those without this history, 0.60% versus 1.93%, respectively (P = .093). Also, there was no difference in the rate of rupture for those patients with a previous normal spontaneous vaginal delivery (1.3%) compared with those without a vaginal delivery (1.6%; P = .76). However, the uterine rupture rate was nearly 10 times higher in the unsuccessful VBAC group (5.4%) than in the successful VBAC group (0.5%; P < .001). This is likely because once uterine rupture was suspected, the patients were taken immediately for cesarean delivery. In this series there were no fetal deaths noted once a uterine rupture had occurred.
Vaginal birth after cesarean delivery is successful in 60–80% of all candidates.4 Our study sought to determine which factors in the maternal history would make VBAC success more likely and found that a history of a previous VBAC made it 7 times more likely to have a successful VBAC. Most experts agree that an important benefit of VBAC is elimination of the need for major surgery. It allows patients wanting large families to have multiple deliveries without the potential for multiple repeat cesareans.5 Women planning further pregnancies avoid the risks of placenta accreta, increased chances of uterine rupture, and the morbidity related to multiple abdominal surgeries that repeated cesarean deliveries can bring.5 The incidence of postpartum infection, need for transfusion, maternal length of stay, and cost are all significantly reduced with VBAC.4 However, when elective repeat cesarean delivery was compared with cesarean delivery after failed trial of labor, the patients who failed attempted VBAC had the highest morbidities.2,6
Recently, the risks of VBAC to the fetus have been readdressed. A meta-analysis in 1991 and a prospective multicenter trial in 1994 both found no increase in perinatal mortality for VBAC versus the overall rate.4 However, one large retrospective trial looking at over 20,000 women with a second delivery after cesarean delivery showed an 11-fold increase in fetal mortality once the uterus ruptured.3 Although the absolute number was 5 fetuses (or 5.5% of all uterine ruptures), the question of acceptable risk thresholds arose.
The potential benefits and harm of VBAC to the mother have also been reviewed.7,8 A recent, systematic literature review by Guise and colleagues8 showed that deficiencies in the literature make it difficult to estimate the risk of VBAC to the mother. This is due in part to a variation in terminology and definitions of uterine rupture and the surrounding complications.
With physicians and patients both acutely aware of the risks and benefits in attempting a vaginal birth after cesarean delivery, identifying those patients who will have VBAC success has become even more crucial. The American College of Obstetricians and Gynecologists has suggested strict criteria in the selection of candidates for VBAC.9,10 This includes limiting VBAC attempts to patients who have had only one previous low-transverse cesarean delivery, have a clinically adequate pelvis, and who do not have a medical or obstetric complication that precludes vaginal delivery.9,10
Careful patient selection to improve chances of VBAC success has become the focus of more recent literature looking at this issue.11,12 A review of 173 patients with a history of one previous low-transverse cesarean delivery in one group's private practice showed an 87% successful VBAC rate.11 The authors attributed this success to careful patient selection. They discouraged patients from attempting VBAC if there was macrosomia or malpresentation in the current pregnancy and also if they had a clinically small pelvis.11 Another review of outcomes associated with VBAC suggested that women with macrosomic infants and those who have never had a successful vaginal delivery should refrain from attempting VBAC.12
There is another body of literature evaluating predictors for uterine rupture. One hypothesis put forth is that a woman who delivers soon after a cesarean delivery is more likely to rupture her uterus. Huang and colleagues13 reviewed 1,516 patients who underwent VBAC and found that an interdelivery interval of less than 19 months was associated with a decreased rate of VBAC success but no increase in rupture. In contrast, Bujold and colleagues14 reviewed 1,527 women attempting VBAC and found that an interdelivery interval of 24 months or less was associated with a 2- to 3-fold increase in uterine rupture but found no difference in VBAC success rates.
Other possible predictors of uterine rupture have also yielded conflicting results. Single- versus double-layer uterine closure and uterine rupture rates were evaluated retrospectively by Bujold et al.15 They found a 4-fold increased rate of uterine rupture in the single-layer closure group. Alternatively, Durnwald and Mercer16 looked at the two types of closure prospectively and found no difference in uterine rupture rates. There is also a suggestion in the literature that increasing infant birth weights are associated with decreasing VBAC success.17
This study evaluated those patients who have had a successful VBAC in an attempt to determine those factors that increase the likelihood of this outcome. We found that a history of a previous VBAC makes a patient 7 times more likely to repeat that success in a future attempt. This outcome was better than that for patients who had just had a previous normal spontaneous vaginal delivery alone. However, when all of the variables were controlled for in the logistic regression model, a history of a previous successful normal spontaneous vaginal delivery no longer significantly influenced future VBAC outcome. An analysis of the data showed that once a patient had a recurrent indication for cesarean delivery, the history of a previous normal spontaneous vaginal delivery no longer led to VBAC success. Induction of labor also had no effect on VBAC outcome.
A birth weight of more than 4,000 g also significantly impacted VBAC success. This was independent of the presence of gestational or pregestational diabetes.
The strength of the present study lies in the large number of VBAC attempts identified in a single institution. However, there are a few notable weaknesses. First, we do not have information on how many patients were initially offered VBAC and declined. Also, we do not have information on the counseling process for patients with a pregnancy following a primary cesarean delivery. This information would help us to evaluate for selection bias. However, because this cohort is from a single center, a wide variation in practices is unlikely. Also, there are currently no protocols in place for determining which patients should be offered a vaginal trial of labor after documentation of a previous low-transverse scar has been established. Finally, the retrospective nature of this study should be pointed out.
With the risks and benefits of vaginal birth after cesarean delivery being debated in both the scientific and the lay literature, we have attempted to re-evaluate VBAC outcomes by looking for factors to prognosticate success. Not all women with a previous low-transverse cesarean delivery make good candidates for VBAC. Identifying which women will be successful can help to decrease perinatal morbidity and mortality. Our data clearly shows that women who have had a previous successful VBAC, those who had a previous cesarean delivery for a nonrecurring indication, and those whose fetuses weighed less than 4,000 g at delivery are more likely to have successful VBAC attempts. Furthermore, although a history of a previous spontaneous vaginal delivery was a strong indicator for VBAC success, the opposite effect of having a recurrent indication for cesarean delivery weighed more strongly on VBAC outcome. These findings should aid the physician in counseling patients who are considering VBAC.
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© 2004 by The American College of Obstetricians and Gynecologists.
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