Introduction
Between 1965 and 1988, the cesarean rate in the United States increased from 4.5% to 24.7%, prompting efforts to minimize unnecessary operative deliveries [1]. Obstetric practice at that time was heavily influenced by the dictum of “once a cesarean, always a cesarean”[2]. Vaginal birth after cesarean (VBAC) was uncommon, and repeat cesarean sections accounted for approximately one-third of all abdominal deliveries. In 1980, the National Institute of Health convened a Consensus Development Conference on Cesarean Childbirth, recommending that a trial of labor (TOL) be considered an option for women with a previous cesarean delivery [1]. With growing support from the American College of Obstetricians and Gynecologists (ACOG), the national VBAC rate increased from approximately 1% in 1970 to a high of 28.3% in 1996, contributing to a fall in the overall cesarean section rate to 20.7% [3]. However, enthusiasm for VBAC was dampened by a number of reports of adverse outcomes related to uterine rupture. Since 1996, the VBAC rate in the United States has been on the decline, reaching a low of 9.2% in 2004 [4]. Due in part to the falling rate of VBAC, the overall cesarean section rate in the United States reached a new high of 31.1% in 2006 [5].
During the 20th century, the classic uterine incision gave way to the low-transverse incision. Early experience suggested that the low-transverse uterine incision conferred a much lower risk of subsequent uterine rupture than did its predecessor. Lavin reviewed the English language literature between 1950 and 1980 and reported the outcomes of 3214 trials of labor in women with previous cesarean sections [6]. The vaginal delivery rate was 66.7%, and 21 women (0.7%) experienced uterine rupture. Since 1980, more than 24,000 trials of labor have been reported in the literature, with a mean success rate of approximately 80%, and uterine rupture rates as low as 0.2–0.8%. In 2004, Guise published a review of the literature from 1980 to 2002 and reported a rate of rupture of 0.38% [7].
The principal anticipated benefits of VBAC include avoidance of the morbidity, mortality and cost of a major abdominal operation. The major risks of VBAC are those associated with intrapartum separation of the uterine scar. With respect to severity, scar separation may be classified as dehiscence or rupture. Uterine scar dehiscence is asymptomatic and is encountered incidentally during uterine exploration following vaginal delivery or at the time of cesarean section for an unrelated indication. Uterine rupture is defined as a defect that: (1) involves the entire thickness of the uterine wall, and (2) is associated with at least one of the following:
- laparotomy for control of hemorrhage from the uterine defect
- hysterectomy due to hemorrhage from the uterine defect
- repair of damage to the uterus or surrounding organs caused by uterine scar separation
- extrusion of any part of the fetus, placenta or umbilical cord through the uterine defect
- cesarean section for acute fetal compromise
Overall, more than 80% of women who attempt VBAC are successful. However, several factors can influence the likelihood of success and the risk of uterine rupture [8].
Type of previous incision
Prospective data are limited but estimated risks of uterine rupture range from less than 1% with a single previous low-transverse uterine incision to approximately 4–9% with a classic or T-shaped incision [8]. With a previous low-vertical incision, reported rates range from 1% to 7% [9].
Number of previous cesareans
With two previous low-transverse cesarean deliveries, reported rates of uterine rupture range from 1% to almost 4%. In one study, the success rate of VBAC was 75% among women undertaking a TOL with two previous cesareans [10]. One or more previous successful vaginal deliveries may reduce the rupture rate and increase the success rate significantly [11].
Indication for previous cesarean
The indication for the previous cesarean section can influence the likelihood of a successful VBAC. In women with previous cesarean sections for “dystocia” or “failure to progress,” reported success rates range from 54% to 77%, with an average of approximately 65%. The risk of uterine rupture does not appear to be increased.
Fetal macrosomia
Although fetal macrosomia may lower the likelihood of successful VBAC, the impact on the rate of uterine rupture may be limited to those women who have not had a previous vaginal delivery.
Maternal age
While some studies demonstrate a lower VBAC success rate in women aged 35 and older, there is no established consensus regarding the impact of maternal age on the risk of uterine rupture [12].
Cervical status
Cervical status prior to induction has been reported to influence the likelihood of successful VBAC as well as the risk of uterine rupture. Among women with modified Bishop scores of 0–2, Bujold reported a successful VBAC rate of 57.8% and a rupture rate of 0.21%, compared to a success rate of 97% and a rupture rate of 0% among those with a Bishop score 9 or greater [13].
Layers of uterine closure
In women with a previous single-layer uterine closure, one study reported a sixfold higher rate of uterine rupture (3.1%) than in women with a previous double-layer closure (0.5%). After correcting for other factors such as prior vaginal delivery, birthweight and interdelivery interval, the risk was still four times higher with a previous single-layer uterine closure [14].
Interdelivery interval
An interdelivery interval of less than 18 months has been reported to increase the risk of uterine rupture threefold. Interdelivery intervals of at least 2 years are associated with uterine rupture rates less than 1%, while rupture rates of more than 4% have been reported when the interdelivery interval is 1 year or less [15]. The impact on TOL success rates has not been established.
Thickness of the lower uterine segment
Although data are limited, one study reported a higher rate of uterine scar separation among women with a sonographically measured lower uterine segment thickness of 2.5 mm or less. A measurement less than 3.5 mm had a positive predictive value for uterine scar separation of 11.8% and a negative predictive value of 99.3% [16]. Lower uterine segment thickness has an unknown effect on the likelihood of successful VBAC.
Dystocia in the current labor
Dystocia is a recognized risk factor for failed VBAC. In at least one case–control study, dysfunctional labor was significantly more common among women with uterine rupture than among controls without scar separation [17]. Arrest of dilation was 10 times more likely in cases than controls.
Oxytocin and prostaglandin
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
