Although the Bishop score, or some adaptation of this basic scoring system, is widely used to predict the outcome of induction of labor, it does not perform well clinically, and more accurate prognostic indicators would be useful. Williams and colleagues compared the accuracy of a cervical score based on cervical ripening with the classic Bishop score in predicting successful induction of labor and vaginal delivery.
The 443 women in the study had initial Bishop scores of less than nine at the time that delivery was required for obstetric or fetal indications. For the first six weeks of the study, scores assessed by the residents were confirmed by attending physicians until uniformity was obtained. Cervical dilation and length were estimated in centimeters. Patients in the study were stratified by gestational age, parity and initial Bishop score, and then randomly assigned to receive either hygroscopic cervical dilation or prostaglandin E2 for cervical ripening. The principal outcomes measured were the onset of active labor within 12 hours of placement of the cervical dilators or prostaglandin, and vaginal delivery.
The two groups of patients were similar with respect to Bishop scores and other measures of cervical status. They were also similar in maternal and pregnancy characteristics. After cervical ripening, the two groups did not differ in Bishop score, cervical consistency or cervical position.
Compared with the Bishop score, cervical dilation was a better predictor of successful labor induction, duration of the latent phase of labor, duration of the second stage, oxytocin use and successful vaginal delivery, both before and after cervical ripening. Successful labor induction was associated with lower maternal weight and body mass index, greater gestation and decreased nulliparity. Vaginal delivery was associated with lower maternal weight and body mass index, and with lower infant birth weight. After stepwise logistical regression, successful induction of labor was most closely associated with preripening cervical dilation. Postripening cervical dilation also correlated more closely with successful induction than other measures, including the Bishop score. Similar results were obtained for the outcome of vaginal delivery.
The authors conclude that cervical dilation is a predictor of successful labor induction and vaginal delivery. The study results indicated that the Bishop score and its component cervical measures, and nulliparity were not significantly associated with these outcomes when adjusted for cervical dilation. The authors advocate the use of simpler measures of cervical status in the assessment of patients for induction of labor.