Can Ultrasound Measurements Replace Digitally Assessed Elements of the Bishop Score?

Abstract

It has been suggested that an objective assessment of cervical ripeness obtained by ultrasound might be preferable to digital assessment using the Bishop score in predicting successful induction. Although the predictive value of both individual ultrasound measurements and elements of the Bishop score appears to be limited, some combination of ultrasound and Bishop score measurements might be more useful. This prospective study compared the utility of cervical measurements obtained by ultrasound imaging before induction of labor with corresponding elements of the digitally determined Bishop score for predicting the probability of successful induction. The study group included 275 women, with singleton pregnancies at ≥37 weeks of gestation and a fetus in cephalic presentation, who were admitted for induction of labor at a university hospital in Norway. Immediately before labor induction, transperineal ultrasound was used to measure the distance from the fetal head to the perineum, the cervical length, posterior angle, and dilation. A Bishop score was then determined digitally by an operator without knowledge of the ultrasound measurements. Receiver operating characteristics (ROC) curves were used to evaluate the probability of a successful labor induction. Univariate analysis showed that predictive factors for successful induction included previous vaginal birth, maternal height, digital assessment of cervical dilatation, and certain sonographic measurements: a fetal head-perineum distance ≤40 mm, cervical length ≤25 mm, and posterior cervical angle >90 degrees (for all factors, P 90 degrees (aOR, 2.6; 95% CI, 1.1-6.1) had only borderline significance. ROC curve analysis showed that a scoring model combining ultrasound measurements of fetal head-perineum distance, cervical length, and posterior cervical angle with digital assessment of cervical dilatation discriminated between successful and failed induction at a curve value of 71 % (95% CI, 61 %-80%, P < 0.01). These findings suggest that a combination of digitally-assessed cervical dilation and ultrasound measurements of fetal head descent, cervical length, and position can predict success of labor induction.