Comparison of Ultrasonographic Cervical Length and the Bishop Score in Predicting Successful Labor Outcomes

Abstract

Previous studies comparing the Bishop score with transvaginal cervical ultrasonography for predicting the results of inducing labor have yielded inconsistent results. The present study compared the performance of these measures and determined the most useful cutoff points in 177 women with singleton pregnancies who were evaluated at 36 to 42 weeks' gestation. All had a live fetus in cephalic presentation. The components of the Bishop score were assessed by digital examination, and a different obstetrician measured cervical length by ultrasound. Labor was induced if the Bishop score was less than 6 and the membranes were intact, using 1 or, if needed, 2 intracervical doses of prostaglandin E2 gel. Oxytocin induction was then begun after 24 hours if required. Oxytocin induction was begun without recording cervical characteristics if premature rupture of the membranes or spontaneous membrane rupture after administration of prostaglandin gel occurred. The commonest indications for induction were prolonged pregnancy and premature membrane rupture. The rate of vaginal delivery in this series was 64%, and delivery took place within 60 hours of initiating induction in all but one of 114 pregnancies. On multiple regression analysis, cervical length, Bishop score, and parity were significantly and independently associated with the likelihood of delivering within 60 hours. The likelihood of vaginal delivery increased as cervical length decreased or the Bishop score increased, and was greater in multiparas than in nulliparas. Although cervical length correlated significantly with the Bishop score, cervical length was a better predictor. The only part of the Bishop score that independently predicted vaginal delivery within 60 hours was station. Based on the area under the receiver operating characteristic curve, the best cutoff points for predicting successful induction of labor were a cervical length ≥24 mm or a Bishop score of ≥4. Estimating cervical length by transvaginal ultrasonography was a better predictor of successful induction of labor than was the Bishop score in this observational study, regardless of whether induction was achieved using topical prostaglandin or oxytocin.