Risk of uterine rupture among women attempting vaginal birth after cesarean with an unknown uterine scar.

OBJECTIVE The purpose of this study was to estimate the association of uterine rupture and previous incision type, either unknown or low transverse, among women who attempt a trial of labor after 1 previous cesarean delivery. STUDY DESIGN We conducted a secondary analysis of a prospective multicenter observational study of 15,519 women with term singletons who attempted a trial of labor after 1 previous cesarean delivery. Odds ratios for the association between uterine incision location, either unknown or low transverse, and uterine rupture were estimated with the use of multivariable logistic regression. RESULTS Between 1999 and 2002, 99 of the 15,519 women (0.64%) who attempted a trial of labor after 1 previous cesarean delivery experienced a uterine rupture. Pregnant women with an unknown scar had lower odds of uterine rupture (adjusted odds ratio, 0.71; 95% confidence interval, 0.37-1.37) compared with women with a known low transverse scar. Other adverse maternal outcomes did not differ between the 2 groups of women. CONCLUSION Among this cohort, women with an unknown uterine incision who attempted a trial of labor were not at increased risk of uterine rupture compared with women with a known low transverse incision.