Examining the Evidence
Does a higher dosage of oxytocin raise the risk of rupture in VBAC candidates?
Yes. A progressive increase in the rate of uterine rupture with increasing oxytocin dosage was observed in this retrospective study in women attempting vaginal birth after cesarean delivery (VBAC), beginning at maximum dosages of 6–10 mU/min (adjusted odds ratio, 1.97) and greatest at the highest range studied, 21–30 mU/min (adjusted odds ratio, 2.98). However, the overall rate of rupture in this study—even at the highest oxytocin dosages studied—was, at 2%, relatively low.
Numerous studies support the safety of a trial of labor after one low-transverse cesarean as an alternative to elective repeat cesarean delivery, with favorable maternal and perinatal outcomes expected for the vast majority of carefully selected patients. Although ACOG acknowledges that oxytocin appears to be relatively safe in patients attempting VBAC, some studies have shown an increased rate of uterine rupture with labor induction and augmentation than with spontaneous labor, suggesting that use of oxytocin may be a risk factor for rupture.1
A focus on maximum dosages
This retrospective study is one of several derived from a large cohort of women with at least one previous low-transverse cesarean delivery performed at one of 17 centers in the United States. Earlier studies from this cohort found that oxytocin alone was not associated with uterine rupture and that no single factor was sufficient to predict rupture.2,3 The majorobjective of this subanalysis was to determine whether higher maximum dosages of oxytocin increase the rate of rupture.
Findings are probably not useful
Despite the strengths of this large study, with observations adjusted for significant confounders, it is limited by its retrospective design, potential bias introduced by nonrandomization, and use of maximum dosage of oxytocin as the primary variable. Many factors are weighed by practitioners when they consider a patient for VBAC and for oxytocin administration, and not all of them could be accounted for in this study: Timing of oxytocin administration, dosing intervals, duration of oxytocin exposure, and total cumulative dosage of oxytocin were not assessed.
Maximum oxytocin dosage was only a fair predictor of uterine rupture, and the authors acknowledge that the maximum dosage of oxytocin is not sufficiently predictive to be clinically useful. Maximum oxytocin dosage is likely only one of the variables affecting the rate of rupture.
Counsel women about greater risk
Although this study contributes to our understanding of uterine rupture in VBAC, its findings do not warrant a change in current clinical practice. The absolute increase of uterine rupture with higher maximum oxytocin dosages was about 1%. Patients should be informed about the possible increased risk of rupture with higher dosages of oxytocin. However, ACOG’s existing recommendations on VBAC should still guide practitioners, and oxytocin should remain an option for properly selected patients in adequately staffed and monitored hospitals.1
1. American College of Obstetricians and Gynecologists. Vaginal birth after previous cesarean delivery. Practice Bulletin#54. Washington, DC: ACOG; July 2004.
2. Macones GA, Peipert J, Nelson DB, et al. Maternal complications with vaginal birth after cesarean delivery:a multicenter study. C Am J Obstet Gynecol. 2005;193:1656-1662.
3. Macones GA, Cahill AG, Stamilio DM, Odibo A, Peipert J, Stevens EJ. Can uterine rupture in patients attempting vaginal birth after cesarean delivery be predicted? Am J Obstet Gynecol. 2006;195:1148-1152.