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When is VBAC appropriate?

Vaginal birth after cesarean carries little risk for mother or infant—provided it is successful. Here’s what the literature says about when you can offer this option.

July 2010 · Vol. 22, No. 07


At first glance, the issue of vaginal birth after cesarean delivery (VBAC) appears to boil down to a simple question: Should I attempt it, or shouldn’t I?

On deeper inspection, the decision becomes extremely complex, and the evidence can be confusing.

Both planned elective repeat cesarean and planned VBAC are associated with harms as well as benefits. Most experts would agree than an uncomplicated vaginal delivery poses little risk to mother and baby, and that a planned repeat cesarean delivery at term carries some risk to the mother.

The greatest risks for both mother and baby arise when a trial of labor fails and cesarean delivery becomes necessary for maternal or fetal indications. Risks to the mother are largely operative in nature, and the primary risk to the fetus is uterine rupture. However, maternal and fetal risks cannot be truly separated. Uterine rupture not only compromises the fetus in utero but has a severe impact on maternal hemodynamic stability, just as a fetal hypoxic ischemic insult secondary to uterine rupture can have lifelong psychological and social consequences for the mother and family.

We are fortunate that serious adverse outcomes of VBAC are rare. Nevertheless, the only predictable delivery method is planned elective repeat cesarean. Uncertainty over the likelihood of success of VBAC arises when relative risk is confused with absolute risk.

In this article, I examine the literature on the route of delivery after cesarean to assess the overall safety of a trial of labor in various settings and populations.

Data on VBAC are limited

We lack randomized, controlled trials and valid animal studies that assess fetal and maternal outcomes of elective repeat cesarean versus planned vaginal delivery. The vast majority of studies of VBAC are retrospective or cohort studies, which have inherent potential for bias. Many studies lack a standardized definition of adverse outcomes or lack direct evidence that adverse outcomes are wholly attributable to the trial of labor. No studies compare women who are similar in all characteristics except their mode of delivery.

Nor do we fully understand how women choose a course of action after cesarean delivery—except that the decision is almost always multifactorial. Competing voices—health care provider, family members, friends, media, and a woman’s own memory of her previous delivery—and her emotional state—all contribute to the decision.


Clearly, a trial of labor after cesarean delivery can be safe for many women. Successful vaginal delivery is associated with a very low risk of adverse outcomes and may be associated with a lower risk of minor morbidity than is elective repeat cesarean. In fact, the overall success rate for a trial of labor after cesarean is not that different from the success rate for nulliparous women undergoing induction of labor.19 Even so, patients should understand that operative delivery may be necessary, and the physician and hospital must be prepared for this eventuality in accordance with ACOG guidelines.

As I interpret the data, if a woman has undergone one low transverse cesarean delivery for a nonrecurring condition and a nonmacrosomic fetus, a trial of labor after the spontaneous onset of labor should be strongly encouraged. If she has already delivered vaginally in the past, or had a successful VBAC, she is an even better candidate for a trial of labor. In such a case, labor induction with mechanical cervical ripening or appropriate use of oxytocin, or both, may still be appropriate, but the likelihood of success is lower.

If a woman has a history of more than one cesarean delivery without a vaginal birth, she may be better served by scheduled repeat cesarean delivery. The same holds true for women who have a history of preterm cesarean delivery, a short interpregnancy interval, suspected macrosomia, or an unengaged fetal vertex.

Decision-making about delivery should be shared between the provider and patient, after thorough counseling about the risks and benefits in language the patient can easily comprehend.

It would be best to avoid having to make a decision about VBAC by preventing the initial cesarean delivery.

How risky is repeat cesarean?

We are all acutely aware of the skyrocketing rate of cesarean delivery, which reaches 35% to 41% in some areas. Most studies indicate that approximately 50% of all cesarean deliveries are repeat cesarean deliveries. Besides the risks associated with the operation itself, planned repeat cesarean has significant downstream implications for the mother and baby—and for society. For example, multiple cesarean deliveries pose an ever greater risk of abnormal placentation and maternal hemorrhage. Cesarean delivery without labor can also heighten the risk of neonatal respiratory compromise, temperature instability, and slow feeding.1 Cesarean delivery and its longer attendant hospitalization markedly increase costs throughout an already strapped health care system.

On balance, any cesarean delivery imparts an increased risk of maternal morbidity and mortality, compared with vaginal delivery, as well as an increased risk of complications, such as placenta previa and placenta accreta, in subsequent pregnancies.

What are the risks of a trial of labor?

A prospective, 4-year observational study conducted at 19 academic medical centers under the auspices of the National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network compared the outcomes of 17,898 women undergoing a trial of labor after cesarean delivery with those of 15,801 women having elective repeat cesarean.2 Symptomatic uterine rupture occurred in 0.7% of the women attempting a trial of labor, with no occurrences in the elective cesarean group. Blood transfusion and endomyometritis were more common in the group undergoing a trial of labor, and this difference was statistically significant. These findings are in concordance with those of earlier studies.

The two groups in this study were not exactly the same; more women undergoing a trial of labor had had a previous vaginal delivery. Significant adverse maternal outcomes, such as endomyometritis, uterine rupture, hysterectomy, and the need for transfusion, were much more likely in a failed trial of labor than in a successful one.

The same study found a 0.46% risk of hypoxic-ischemic encephalopathy, which was most likely to occur after symptomatic uterine rupture (7 of 12 cases). No cases of hypoxic-ischemic encephalopathy occurred among women undergoing planned cesarean delivery. Multivariate logistic regression analysis determined that the risks of stillbirth, neonatal death, and hypoxic-ischemic encephalopathy in term infants were increased in the group undergoing a trial of labor, compared with elective repeat cesarean (odds ratio [OR], 2.72; 95% confidence interval [CI], 1.49–4.97).

Can we predict the success of a trial of labor?

Combined success rates from a large number of prospective cohort studies suggest an overall rate of 75.9%. Many clinical characteristics may increase the likelihood of success of a trial of labor after cesarean. In this section, I describe these characteristics and sift the data we have about them.

A history of vaginal delivery ups the odds of success

Women who have delivered vaginally have a much lower risk of rupture during a trial of labor after cesarean than women who have not. Women who have delivered vaginally are also four times more likely to have a successful VBAC. A multicenter, prospective study found a VBAC success rate of 86% among women who had already delivered vaginally, and a success rate of 90% among women who had a history of successful VBAC.3

Many aspects of the cesarean delivery have continuing impact

The type of labor that occurred in the cesarean delivery may help predict subsequent complications and the ultimate success of a trial of labor. For example, induced labor or no labor prior to cesarean delivery is associated with a 2.25-fold risk of uterine rupture in a subsequent trial of labor, compared with a history of spontaneous labor.4

In addition, several studies have demonstrated that the indication for the first cesarean delivery has a bearing on the success of a subsequent trial of labor. For example, an indication of shoulder dystocia reduces the success rate of a subsequent trial of labor by one third.2

Even a brief trial of labor before the cesarean may increase the success of a subsequent trial of labor. One study found that cervical dilation to 8 cm or greater was independently predictive of successful VBAC among women who had a nonrecurring indication for the initial cesarean delivery.5

When the cesarean delivery involves a preterm infant, the risk of uterine rupture during a subsequent trial of labor may increase if the infant is at term. Conversely, the risk of uterine rupture is lower when a term cesarean is followed by a preterm trial of labor.6

A vertical hysterotomy may preclude VBAC

A previous classical hysterotomy is generally an absolute contraindication for a trial of labor because rupture may occur in as many as 14% of women who have this type of scar.

Low transverse hysterotomy does not appear to confer excess risk during a subsequent trial of labor. Less clear is whether a low vertical hysterotomy poses a risk of rupture. In a 2004 prospective cohort study, the rate of uterine rupture among women who had a transverse hysterotomy scar was 0.7%, compared with 2.0% for a low vertical scar. Any difference in the rate of uterine rupture in retrospective studies may be attributable, at least in part, to the subjective nature of the definition of “low vertical” because there is no precise or objective way to ensure that the vertical hysterotomy did not breach the contractile portion of the uterus (FIGURE).

Type of prior hysterotomy influences the VBAC decision

A classical uterine incision is an absolute contraindication to vaginal birth after cesarean (VBAC). A trial of labor is thought to be safe in women who have had a low transverse hysterotomy. The jury is still out on the safety of VBAC in a woman who has had a low vertical incision, however, because of uncertainty over whether the contractile portion of the uterus is involved.

Are multiple cesareans a contraindication to VBAC?

Experts disagree as to whether more than one previous cesarean delivery before a trial of labor increases the risk of uterine rupture. One retrospective study showed no difference in the rate of rupture between women who had a single previous cesarean and those who had more than one.7 A larger prospective study showed a modest increase in the risk of rupture (OR, 1.16) among women who had undergone more than one cesarean—but no decrease in the chance of success.8

Most large retrospective and prospective studies include patients who have had more than one previous cesarean delivery, but their numbers remain low; therefore, statistical significance cannot be determined.

Induction or augmentation of labor may lower odds of success

The likelihood of successful VBAC may be reduced when labor is augmented or induced. The picture is unclear because most studies that have focused on cervical ripening and induction of labor in VBAC are small.

Bujold compared pregnancy outcomes of three groups of women:

  • those who underwent cervical ripening via Foley catheter
  • those who had amniotomy and oxytocin administration
  • those who entered labor spontaneously.

No difference in the rate of uterine rupture was found among the groups. However, the group that underwent cervical ripening had a significantly lower rate of success.9

A large case-control study found no increase in the rate of rupture when oxytocin or prostaglandins were administered, but the rate tripled when both were used together.10

A small, nested, case-control study found an increased risk of uterine rupture only when oxytocin was administered at a rate exceeding 20 mU/mL.11

More than 90% of hysterotomies are transverse

When the obstetric history is incomplete, the clinician may not know what type of hysterotomy was used in the previous cesarean delivery. Most experts believe that VBAC is acceptable when the previous cesarean involved a low transverse hysterotomy. The risk may be much higher with other types of incisions. Today, however, with modern techniques in place, we can assume that more than 90% of hysterotomies are of the low transverse type.

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