The Bishop score components correlated to sonographic measurements prior to induction of labor

Abstract

Objective: The Bishop score remains the gold standard for assessing favourability for induction of labor, but it is a subjective evaluation with limitations. The aims of this prospective study were to relate the components of the Bishop score to corresponding sonographic measurements and to assess predictive values for a successful labor induction. Methods: In 275 women the fetal head-perineum distance was measured through a transverse transperineal scan, and the cervical length, posterior cervical angle and cervical dilatation through a transvaginal scan. The Bishop score was assessed without knowledge of ultrasound measurements immediately after the scans. Correlation analyses were done, and receiver-operating characteristics (ROC) curves were used for evaluation of the probability of a successful vaginal delivery. Results: By sonographic assessment the cervix was closed in 219 (80%) of the women compared to 58 (21%) by digital assessment. Spearman\'s correlation coefficient for digital and ultrasound assessment of cervical length was 0.54 (p<0.01), fetal head decent 0.23 (p<0.01) and cervical position/angle 0.03. The predictive value for a vaginal delivery after induction of labor for Bishop score and ultrasound measurements is presented in figure 1. The best predictive factors for a vaginal delivery were digital assessment of cervical dilatation with 61%; 95% CI 51–71% (p=0.03) under the ROC curve area, and a combination of ultrasound measured fetal head-perineum distance, cervical length and cervical angle with 67%; 95% CI 56–77% (p<0.01) under the curve area. Conclusion: The correlation between ultrasound and digital assessments is weak. None of the factors used alone are good predictors of labor outcome. Combinations of factors improve the prediction.