An ultrasound prediction model for probability of vaginal delivery in induction of labor

Abstract

Objective Our aim was (1) to evaluate a pre-induction ultrasound score for prediction of successful induction of labour by attaining active phase of labour and vaginal birth and compare it with the Bishop score in term nulliparous women, and (2) to formulate a prediction model to calculate probability of vaginal delivery for clinical use. Methods 96 nulliparous women between 36-41 weeks gestation were recruited. All subjects fulfilled the inclusion criteria of a live singleton pregnancy, vertex presentation, intact amniotic membrane, in the absence of active labour with no contraindication to vaginal delivery. The patients were assessed by our ultrasound score comprising of 3 cervical and 2 fetal head parameters. These parameters were fetal head position, fetal head symphysis pubis distance relation, cervical length, funneling and posterior cervical angle. Each parameter was scored from 0-2, with a maximum score of 10. A printout of each ultrasound study was produced, and measurements were performed using a scale and a protractor. The final ultrasound score was calculated by using the below pelvic ultrasound scoring system. Table 1 Parameters of pelvic ultrasound score Score 0 1 2 Cervical length ≥3.5 CM 2.1-3.4 ≤2 Funneling Absent - Present Posterior cervical angle ≤900 91-109 ≥110 Fetal head position OP OT OA Fetal head symphysis distance relation Measurable Touching Not Measurable A second obstetrician blinded to the sonographic findings assessed the modified Bishop score and recorded the findings. We calculated sensitivity, specificity, FPR, PPV, NPV and accuracy were calculated for both events, achieving active phase of labour and vaginal delivery. SPSS 20 was used for ROC curves plots and calculation of area under curve. Binary Logistic Regression model was prepared and probability of vaginal delivery for various scores was calculated. Results Out of 91, 61(67%) achieved active phase of labour and 54(59%) had vaginal delivery. Our pelvic ultrasound score showed better sensitivity and specificity for both achieving an active phase of labour and vaginal delivery in comparison to the Bishop score. At a cut off of ≥ 5, the ultrasound score showed sensitivity of 79.3 %, specificity of 75.8 % for achieving vaginal delivery. Whereas, the Bishop score showed sensitivity of 66.7 % and specificity of 44.2 % respectively. A binary logistic regression model predicted 78.0% of the events correctly. Other consultants of hospital not involved in the study, also utilised our model and found it easily reproducible in successful prediction of probability of mode of delivery. Conclusion Our study shows that Garg Ultrasound Score can predict success of induction of labour. This proposed pelvic ultrasound score, if validated in larger multicentre studies, could help clinicians provide evidence-based counselling for predicting probability of vaginal delivery. This in turn, may allow women make a more informed decision before undergoing induction of labour.