Preinduction sonographic measurement of cervical length in the prediction of successful induction of labor

Background Induction of labor is carried out in approximately 20% of pregnancies. However, approximately 20% of women having induction of labor end up having a Cesarean delivery. The traditional method of predicting whether an induced labor will result in successful vaginal delivery is based on the preinduction,'favourability' of the cervix as assessed by the Bishop score. However, this assessment is subjective and several studies have shown a poor predictive value for the outcome of induction. Objectives To examine the relationship between preinduction sonographically measured cervical length and the Bishop score and to compare the two measurements in the prediction of successful vaginal delivery within 24 h of induction. Methods In this multicenter study, preinduction cervical assessment was undertaken in 240 women with singleton pregnancies at 37-42 weeks of gestation. The Bishop score was assessed by digital examination and the cervical length was measured by transvaginal sonography. Results Multiple regression analysis demonstrated that cervical length, Bishop score and parity provided independent contribution in the prediction of the likelihood of delivering vaginally within 24 h. Further examination of the different components of the Bishop score showed that only cervical length provided a significant contribution in the prediction of the likelihood of vaginal delivery within 24 h. In the receiver operating characteristic curves, the best cut-off point for the prediction of successful induction was 28 mm for cervical length and 3 for the Bishop score. However, cervical length appears to be a better predictor than the Bishop score, with a sensitivity of 0.87 and a specificity of 0.71 compared to 0.58 and 0.77, respectively. Similarly, the Kaplan-Meier survival curves indicate that better discriminatory results in the prediction of vaginal delivery within 24 h are achieved using cervical length rather than the Bishop score. Conclusion Transvaginal sonographic measurement of cervical length provides a useful prediction of the likelihood of vaginal delivery within 24 h of induction.