Clinical Inquiries

How safe is vaginal birth after cesarean section for the mother and fetus?

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Compared with planned repeat low-transverse cesarean section, vaginal birth after cesarean section (VBAC) is not associated with increased risk of maternal or neonatal mortality (strength of recommendation [SOR]: B). Morbidity is slightly increased, as evidenced by higher uterine rupture rates and some neonatal outcome measures (SOR: B).


Risks of C-section and labor must be considered when counseling regarding route of delivery
Lynda DeArmond, MD
Waco Family Practice Residency Program, Waco, Texas

Another question to pose is: how safe is repeat cesarean section for the mother and fetus? How much do morbidity and mortality increase with each new intra-abdominal procedure? Each time the belly is opened there is new scar, with increased likelihood of adhesions and potential for future bowel obstruction. Consider these risks when counseling regarding route of delivery. Risk of uterine rupture appears to be higher in trials of labor (and confers a statistically significant but small increase in morbidity but not mortality). However, the uterine scar can silently fail without labor—as is sometimes discovered at a scheduled repeat section, usually without untoward effects on mother or fetus.

Remember that you are sending a young woman home with a new baby to care for (along with other children) and a major abdominal procedure (through an old scar) to recover from, which one could certainly define as morbidity. Cesarean section is an important tool, but we must be careful to practice best possible care and consider all patient factors and preferences. And data are still lacking to support the notion that VBAC is unsafe.

Evidence summary

Contrary to the goals of Healthy People 2010, the rate of cesarean sections is increasing.1 The repeat cesarean rate for low-risk women of all ages and racial groups is now 88.7%, the highest rate since the Centers for Disease Control and Prevention (CDC) began tracking the statistic in 1989. Is VBAC safe, or is a trial of labor no longer supported by the data?

The most recent Cochrane Review found that both VBAC and repeat lowtransverse cesarean section have benefits and risks associated with them; however, after reviewing the limited data, they concluded that no trial exists to adequately help women and their caregivers make an informed decision between the two.2 A strong theme in the Cochrane Review, echoed in most reviews, was the absence of high-quality prospective randomized data.

In an attempt to quantify the risks of VBAC, a systematic review determined that attempted VBAC, compared with repeat low-transverse cesarean section, increased the risk of uterine rupture by 2.7 per 1000 cases (95% confidence interval [CI], 0.73–4.73).3 This additional risk rate is often quoted in VBAC reviews and was cited in the Agency for Healthcare Research and Quality evidence report; it is based on 1 prospective, nonrandomized cohort trial and 1 retrospective cohort study.4,5

No randomized controlled trials exist for determining maternal safety of VBAC, although another recent systematic review found 2 nonrandomized prospective trials of sufficient quality to analyze. The authors concluded there were “no statistically significant differences between planned elective repeat cesarean section and planned VBAC.”6 Upon closer review in PubMed, one of the cited studies did not study 312 patients for VBAC outcomes as alleged; rather, it investigated patient attitudes towards VBAC.7

Since publication of that review, a large, multicenter, prospective, nonrandomized trial involving 33,699 patients found no significant difference between VBAC and planned cesarean for hysterectomy (0.2% vs 0.3%; odds ratio [OR]=0.77; 95% CI, 0.51–1.17), maternal death (0.02% vs 0.04%; OR=0.38; 95% CI, 0.1–1.46), and neonatal death (0.08% vs 0.05%; OR=1.82; 95% CI, 0.73–4.57).8 Significant associations were found for uterine rupture rates in spontaneous labor (24/6685 [0.4%] vs no cases; number needed to harm [NNH]=279) and neonatal hypoxic-ischemic encephalopathy (0.46 cases per 1000 vs no cases; NNH=2174).8

A retrospective Canadian cohort trial of 308,755 women also demonstrated an association of VBAC with uterine rupture(0.65% of trial-of-labor cases; OR=2.38; 95% CI, 2.12–2.67), and a trend towards higher maternal mortality in the cesarean group (1.6 per 100,000 for VBAC vs 5.6 per 100,000 for planned cesarean; OR=0.32; 95% CI, 0.07–1.47).9

The effect of VBAC on neonatal morbidity and mortality is unclear. In contrast to the negative larger trial,8 a smaller retrospective cohort of 24,529 births found a higher association of perinatal death for trial of labor (adjusted OR=11.7; 95% CI, 1.4–101.6).10 The perinatal death rate was similar to rates in nulliparous women. Regarding morbidity, one retrospective cohort trial showed VBAC was associated with an increase in neonatal sepsis (1% vs 0%; CI not given) compared with planned cesarean, but VBAC resulted in less transient tachypnea (5% vs 7%) and hyper-bilirubinemia (2% vs 6%).11

Evidence-based answers from the Family Physicians Inquiries Network

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