Population adjustment of the definition of the vaginal birth after cesarean rate.

OBJECTIVE: The vaginal birth after cesarean delivery rate is calculated with a denominator equal to the number of all women who give birth after a previous cesarean delivery, including those who are not candidates for a trial of labor. We evaluated the impact of adjustment for noncandidates for a trial of labor on vaginal birth after cesarean delivery rates. STUDY DESIGN: All women with a previous cesarean delivery who were delivered during 1998 were classified as either candidates or noncandidates for a trial of labor. An adjusted vaginal birth after cesarean delivery rate was calculated by eliminating noncandidates for a trial of labor from the denominator. The percentage of noncandidates for a trial of labor, the vaginal birth after cesarean delivery rate, and the adjusted vaginal birth after cesarean delivery rate were compared among 3 clinical services. RESULTS: The maternal-fetal medicine service had a significantly higher percentage of noncandidates for a trial of labor than did either the low-risk resident clinic or the low-risk private service. The maternal-fetal medicine service had a significantly lower vaginal birth after cesarean delivery rate than did the private service, but this difference was no longer present after application of an adjusted vaginal birth after cesarean delivery definition. CONCLUSION: For accurate comparison of vaginal birth after cesarean delivery rates among providers it is essential to account for patient risk status in the vaginal birth after cesarean delivery definition through the elimination of noncandidates for a trial of labor.