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Clinical Reviews

VBAC: Safer than you think

The pendulum continues to swing between these 2 delivery options. Has it swung too far toward elective cesarean?

August 2002 · Vol. 14, No. 8


  • Although the risk of uterine rupture and fetal complications may be slightly increased with a trial of labor (TOL), the overall incidence of these complications is low.
  • In a recent meta-analysis involving 47,682 women, a TOL produced more favorable maternal outcomes than elective repeat cesarean (ERC). Women choosing TOL also were much less likely to undergo hysterectomy than those selecting ERC.
  • Many investigators remain reluctant to recommend induction of labor in the setting of vaginal birth after cesarean section (VBAC), fearing an increased risk of uterine rupture when oxytocic agents are used.
  • Between 374 and 809 women would need to undergo ERC to prevent 1 uterine rupture, and between 693 and 3,332 women would need to undergo ERC to prevent 1 perinatal death attributable to a TOL.

Despite numerous studies detailing the safety and efficacy of attempted vaginal birth after cesarean (VBAC), the strategy remains controversial. Many obstetricians are retreating from the assumption that this mode of delivery is safer than elective repeat cesarean (ERC) for most women with 1 or 2 prior cesarean sections. This shift in attitude springs in part from a decreased societal tolerance of risk and in part from a misinterpretation of current data.

Here, I review a large body of literature supporting the contention that a trial of labor (TOL) yields a more favorable maternal risk profile than ERC. Although the risk of uterine rupture and fetal complications may be slightly increased with a TOL, the overall incidence of these complications is reassuringly low.

Absolute risks and benefits

Research into the relative safety of a TOL after cesarean was conducted throughout the 1970s and 1980s. In 1989, Meehan and Magani published data from 15 years of experience at the University College Hospital in Galway, Ireland.1 This series included 1,350 trials of labor, with an 81.26% vaginal delivery rate. Among the women who labored, the incidence of true uterine rupture (which the authors defined as “complete uterine scar disruption, requiring repair at emergency cesarean section or laparotomy”) was 0.44%. In comparison, the incidence of true rupture among the 1,084 women who opted for ERC was 0.37%. There were 4 perinatal deaths attributable to uterine ruptures; 3 occurred in the TOL group, and 1 occurred in the ERC group.

Other evidence from large databases includes a meta-analysis by Rosen and Dickinson, which pooled data from studies carried out in the United States between 1982 and 1989.2 Among the 29 studies included in the analysis, the rate of successful vaginal delivery ranged from 54% to 89%.

A later meta-analysis by Rosen et al compared morbidity and mortality for TOL and ERC.3 In 5,463 trials of labor, there were 22 true uterine ruptures (4/1,000), with 3 perinatal deaths (5/10,000) attributable to these ruptures. There was one maternal death in each group, yielding maternal mortality rates of 2.8 in 10,000 for women choosing a TOL and 2.4 in 10,000 for women undergoing ERC. Maternal febrile morbidity was greatest among women having failed a TOL, intermediate among women undergoing ERC, and lowest among women having successful TOLs. This analysis did not compare maternal morbidity according to intended mode of delivery.

Subsequently, several large series compared TOL with ERC. One from California prospectively compared these delivery options among women receiving obstetrical care within the Kaiser Permanente managed-care organization.4 This cohort study included 5,022 women who attempted vaginal birth, and 2,207 women who underwent ERC. Among women attempting vaginal birth, 75% were successful, with a rate of uterine rupture of 0.8%. There were no perinatal deaths due to rupture, and women attempting vaginal birth required significantly fewer transfusions and had significantly less postpartum fevers than those undergoing ERC. There were fewer hysterectomies among women in the TOL group than the ERC group, though the difference was not statistically significant.

Febrile morbidity was less common among women having a TOL than an ERC.

In another series, Miller et al reported a prospective evaluation of TOL and ERC among 17,322 women with at least 1 previous cesarean who delivered at the University of Southern California Women’s Hospital or at Los Angeles County Hospital.5 Of these women, 12,707 had trials of labor, with 82% delivering vaginally. There were 95 uterine ruptures (0.7%), but the rate of related perinatal death was only 2 in 10,000.

More recently, Rageth and colleagues evaluated 29,046 deliveries after previous cesarean in a pooled Swiss database of 457,825 deliveries.6 Of these patients, 17,613 underwent a TOL, with a success rate of 73.7%. There were 70 uterine ruptures among the women attempting vaginal birth (0.4%) and 22 ruptures among those undergoing ERC (0.2%). Perinatal death was more common among those undergoing a TOL, but the absolute risk of perinatal death was low (2/1,000 for TOL compared with 1/1,000 for ERC when infants with congenital anomalies or extreme prematurity were excluded). Hysterectomy, febrile morbidity, and thromboembolic complications all were significantly less common among women having a TOL than those choosing elective repeat cesarean.

The assumption that elective repeat cesarean will result in significantly fewer cases of long-term neurologic impairment is unproven at this time.

Similarly, Gregory et al reported on a cohort of 66,856 women with prior cesarean deliveries whose records were gathered from 1995 discharge data from the California Office of Statewide Health Planning and Development.7 In this cohort, 39,096 women attempted vaginal delivery, and 61.4% were successful. There were 209 uterine ruptures among women having a TOL (0.5%) and 79 ruptures among those having ERC (0.3%).

Perhaps the most influential recent investigation is a population-based longitudinal study by McMahon and colleagues suggesting that maternal morbidity might be greater with a TOL than with ERC.8 This study included 6,138 women with prior cesarean deliveries. Of these, 3,249 had a TOL, and 2,889 underwent ERC. There were 10 uterine ruptures (0.3%) among those in the TOL group, and 1 uterine rupture (0.0%) among those in the ERC group. There were no significant differences in hysterectomy, puerperal fever, or the need for transfusion. However, operative injuries were significantly more common among women having a TOL, while abdominal wound infections were significantly more frequent among those undergoing elective repeat cesarean.

McMahon et al classified uterine ruptures, hysterectomies, and operative injuries as “major complications,” and puerperal fever, transfusions, and abdominal-wound infections as “minor complications.” They found that pooled major complications were significantly more frequent in the TOL group, but that there was no difference between groups in pooled minor complications. This finding contradicted much earlier research, which suggested maternal morbidity would be reduced when a TOL was undertaken.

This influential study contributed greatly to the decrease in enthusiasm for a TOL. However, a careful examination of its data reveals that though there was greater risk of uterine rupture among women experiencing labor, that number was a quite low 0.3%. And while the difference was not significant, fewer hysterectomies were performed in the TOL group than in the ERC group. Overall maternal morbidity did not differ between the groups. Further, the classification of operative injury as a major complication and the need for blood transfusion as a minor complication is, at least, debatable.

Cutting the legal risks of VBAC

Medical liability claims spurred by complications associated with vaginal birth after cesarean (VBAC) are a disturbing fact. Although the risks of VBAC generally are very low, foremost among them is uterine rupture, which can have dire consequences for both mother and infant.

Of course, when a trial of labor (TOL) is successful—as it usually is—maternal morbidity is lower than with elective repeat cesarean (ERC). For this reason, properly selected and counseled patients should be allowed a TOL if they desire. Other recommendations to help minimize the possibility of litigation include:

Know the risks. As mentioned above, there is a low but significant risk of uterine rupture. In addition, placenta previa and placenta accreta are more likely to occur in women with a history of primary cesarean. If the TOL is unsuccessful, the likelihood of maternal and fetal complications increases further.1 Contraindications to a TOL include a previous uterine rupture, a prior classic or T-shaped uterine incision, a contracted pelvis, and maternal or fetal conditions that preclude vaginal delivery.1

Select patients carefully. Candidates for a TOL include women who have undergone a previous low-transverse cesarean and have no evidence of fetopelvic disproportion.1 Even women who have undergone up to (but not more than) 2 previous cesareans may be allowed a TOL, provided they have no other uterine incisions or contraindications to vaginal delivery. However, they should be counseled that the risk of uterine rupture is greater when there is more than 1 previous incision.

Assess the incision. If the previous incision was low transverse, and no other contraindications are present, the risk of rupture is 0.2% to 1.5%. Other incisions carry a significantly greater risk. These include low vertical (1% to 7% risk), T-shaped (4% to 9%), and classical uterine scars (4% to 9%).1

Appeal global mandates. Some insurers require all women with a previous cesarean delivery to undergo a TOL. Unfortunately, such policies can lead to attempted VBAC in cases where ERC is indicated.1 If a TOL would be unwise for your patient, bring her to the insurer’s attention rather than adhere to potentially harmful requirements.

Be conservative. Adopt a cautious approach in obstetric situations in which TOL is controversial, such as gestational diabetes, multiple gestation, postdate pregnancy, and suspected macrosomia.1,2

Ensure back-up. The obstetrician should offer a TOL only when he or she can ensure immediate access to surgical facilities for emergent cesarean, including skilled health-care personnel, anesthesia, pediatric specialists, and the proper instrumentation. When these are not available, the patient should undergo ERC or be transferred to a hospital that can provide them.1,3

Be vigilant. Continuous fetal monitoring is recommended. Support staff should be well educated about the signs of uterine rupture (nonreassuring fetal heart rate [FHR], abdominal pain, vaginal bleeding, hypovolemia, or a loss of station of the presenting part), and the obstetrician should remain nearby until the infant is delivered. If FHR tracings indicate a long deceleration to 60 to 70 bpm or severe and unresponsive variable decelerations, the obstetrician should intervene immediately. Note that epidural analgesia rarely obscures the signs of rupture.1,3

Write it down. In a number of cases, physicians have had to defend their actions in court based on their memory of how the delivery proceeded, since documentation in the patient’s chart was sparse. The solution? Write everything down. It’s better to have a thorough record and not need it than to need documentation that doesn’t exist.—ELLEN MOZURKEWICH, MD, MS


1. American College of Obstetricians and Gynecologists. Vaginal birth after previous cesarean delivery. Practice Bulletin #5. Washington, DC: ACOG; 1999.

2. Coleman TL, Randall H, Graves W, et al. VBAC among women with gestational diabetes. Am J Obstet Gynecol. 2001;184(6):1104-1107.

3. Flamm BL. Vaginal birth after cesarean: reducing medical and legal risks. Clin Obstet Gynecol. 2001;44(3):622-629.

Sorting recent data

To obtain more precise estimates of morbidity and mortality risks, Eileen Hutton and I performed a meta-analysis of the literature published between 1989 and 1999.9 We included studies in which women undergoing a TOL and those choosing ERC had both been candidates for vaginal birth. That is, we tried to evaluate studies in which those choosing either of the 2 treatments were as comparable as possible at the outset. Unfortunately, in the absence of randomized treatment assignments, it is impossible to exclude all intrinsic differences.

Overall, a TOL results in a more favorable maternal risk profile than ERC.

Fifteen studies involving a total of 47,682 women met our inclusion criteria. We evaluated 8 outcomes of interest: the rates of vaginal birth, uterine rupture, perinatal death, maternal death, maternal febrile morbidity, maternal blood transfusion, hysterectomy, and 5-minute Apgar scores less than 7.

Of the 28,813 women undergoing a TOL, 72.3% achieved vaginal birth. Although the risk of uterine rupture among those choosing TOL was about twice that of those choosing ERC, the absolute risk of this complication was quite small for both groups (0.4% and 0.2%, respectively). The risk of perinatal death was significantly increased in the TOL group, compared with the ERC group; however, the unadjusted risk of death was quite low (0.6% for TOL compared with 0.3% for ERC). When deaths attributable to extreme prematurity or lethal anomalies and intrauterine deaths before labor were excluded, the risk of death decreased to 0.2% in the TOL group and 0.1% in the ERC group. Thus, we calculated the risk of perinatal death attributable to trial of labor to be about 1 in 1,000.

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