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Expectant mothers who have had a previous cesarean delivery may choose a repeat cesarean for the subsequent birth or they may elect trial of vaginal birth after cesarean (VBAC). In population-wide, retrospective cohort study, VBAC was associated with small but significant increases in absolute risk of perinatal death and uterine rupture compared to repeat cesarean (JAMA 2002 May 22-29;287(20):2684). That and other retrospective analyses have been criticized for failing to include truly comparable groups in which all women were eligible for VBAC. Few prospective studies comparing the 2 approaches have been conducted. A new study prospectively compared planned VBAC to planned repeat cesarean in a cohort of 2,345 pregnant women (mean age 31 years) with 1 previous cesarean delivery who were all eligible for VBAC at term.
Women were assigned by patient preference to either VBAC (52.7%) or repeat cesarean. Additionally, 22 women who expressed no preference were randomized to 1 of the 2 approaches. Mean gestational age at study entry was 37.5 weeks. VBAC was successful in 43% of the women in that group. The rest had cesarean delivery (97% of the cesarean group had cesarean delivery).
The assigned and randomized groups were combined for primary analyses. The risk of major maternal hemorrhage (defined as blood loss > 1500 ml or need for transfusion) was significantly increased in the VBAC group (2.3% vs. 0.8%, p = 0.011, NNH 67) (level 3 [lacking direct] evidence). There were no significant differences in any other maternal outcomes.
For the infants, serious neonatal morbidity was defined as a composite of clinical and nonclinical outcomes including birth trauma, seizures, Apgar score ≤ 4 at 5 minutes, and other adverse events. This outcome occurred in 2.3% of the VBAC group and 0.9% of the planned cesarean group (p = 0.014, NNH 72). However, no individual components of the composite showed significant differences. The increase associated with VBAC was due primarily to trends toward increased risk for 2 markers of neonatal asphyxia: cord blood base deficit ≥ 12 mmol/L (p = 0.06) and cord pH < 7 (p = 0.13) (level 3 [lacking direct] evidence). Serious clinical outcomes were too rare for a study of this size to either demonstrate or exclude clinically important differences. There were no significant differences in perinatal death (0.2% vs. 0%). No adverse outcomes were reported in any of the randomized patients (PLoS Med 2012 Mar;9(3):e1001192).
Although cohort studies are more prone to bias than randomized trials, the small but significant differences found in this study may be of value to clinicians when advising women on their birth options. Given that only 0.94% of women had no preference and agreed to be randomized, it may be difficult to reach a definitive answer to the question of absolute risk with VBAC from a randomized controlled trial.
For more information, see the Trial of vaginal birth after cesarean (VBAC) topic in DynaMed.