Increasing the Odds for Success With VBAC

Considering that the risks of pregnancy and childbirth overall are often underappreciated, it is important to share these data with patients and explain that the risks of VBAC are similar in magnitude to complications observed with any vaginal delivery. Certainly, these large observational studies—which provide a broader, more representative look at outcomes than prior studies—provide short-term safety evidence that overall favors VBAC as a standard part of practice.

Selecting Candidates

Patient selection is important, as most of the major complications in women who attempt a trial of labor occur in association with a failed VBAC attempt.

At least several investigators, myself included, have attempted to develop models or scoring systems to predict which women are most likely to be delivered vaginally with a VBAC attempt. Many of these models have incorporated factors that can be ascertained early in prenatal care as well as those that are not known until admission for delivery. Other models focus on factors available at the first prenatal visit, such as maternal age, prepregnancy body mass index, ethnicity, and prior vaginal delivery.

Unfortunately, these models have not been shown to accurately predict who is going to succeed and who is going to fail in a VBAC attempt.

Thus far, the one clinically useful predictive factor we have for VBAC success is prior vaginal delivery, whether it's a prior successful VBAC attempt or a vaginal delivery that predated a cesarean section. Indeed, numerous studies have supported the predictive value of a prior vaginal delivery.

In 2005, for instance, the MFMU reported that a previous vaginal delivery was the most significant predictor of VBAC delivery success in a cohort of 29,661 women with a history of one prior cesarean delivery. Women with a prior vaginal birth had a VBAC delivery success rate of 86.6%, compared with 60.9% in women without a prior vaginal delivery (Am. J. Obstet. Gynecol. 2005;193:1016-23).

A secondary analysis of our large, retrospective observational study on maternal complications with VBAC (discussed above) similarly showed that VBAC candidates with a prior vaginal birth were significantly more successful in achieving vaginal delivery than women with no prior vaginal delivery. The success rate was 89.9%, compared with 67% (Am. J. Obstet. Gynecol. 2006;195:1143-7).

Women with a history of vaginal delivery also appear to have lower rates of major complications, making a VBAC attempt safer in these patients than a planned repeat cesarean section (whether the attempt is successful or not). In our observational study, a prior vaginal delivery was associated with significant reductions in major morbidity.

Clearly, not all women with a history of cesarean delivery are the same, and women with a prior vaginal delivery should be counseled about their more favorable benefit-risk ratio.

Overall, the vaginal delivery rate after a trial of labor is high in women who have had prior cesareans. In our large observational study, the vaginal delivery rate among those women who attempted VBAC was 75.5%. Furthermore, in the draft of its consensus development conference statement, the NIH panel reported that there is a “high grade of evidence” showing that a trial of labor is successful in nearly 75% of cases.

Even in the least favorable groups—among women who might appear to have unfavorable risk profiles for VBAC attempts—the success rate for VBAC is consistently higher than 50%.

Intrapartum Management

We can make a relatively safe and reasonable process even safer by carefully and conservatively managing the intrapartum period in women attempting VBAC.

Here are several tips for managing a trial of labor after cesarean:

▸ Induce labor only when absolutely necessary. Research from both large observational studies on a trial of labor after cesarean has shown that the risk of uterine rupture is two- to threefold higher in women who have their labor induced than in women who are delivered spontaneously. We should therefore refrain from inducing labor unless we have solid medical reasons to do so.

▸ Try to avoid the use of multiple induction agents. If you're considering induction for a VBAC candidate who has an unfavorable cervical exam, reconsider it. Research has also shown that women who require multiple agents for induction have the highest rates of uterine rupture—rates that are almost four- to fivefold higher than those for women who labor spontaneously.

▸ Avoid higher doses of oxytocin. There does not appear to be an increased risk of rupture with oxytocin augmentation of spontaneous labor—unless the dose is in excess of 20 mU/min. An analysis by Dr. A.G. Cahill (Am. J. Obstet. Gynecol. 2007;197:495.e1-5), for example, found a dose-response relationship of maximum oxytocin administration and uterine rupture. Some institutions have already decided not to go above this amount in women attempting VBAC.