Objective To create and externally validate a predictive model to calculate the likelihood of vaginal delivery after preterm induction with unfavorable cervix. Methods This was a retrospective cohort study of women with a singleton gestation from a single academic institution who underwent an induction of labor at less than 37 weeks of gestation from January 2009 to June 2018. Women with contraindications for vaginal delivery were excluded. Analyses were limited to women with unfavorable cervix (both simplified Bishop score [dilation, station, and effacement: range 0-9] less than 6 and cervical dilation less than 3 cm). A stepwise logistic regression analysis was used to identify the factors associated with vaginal delivery by considering maternal characteristics and comorbidities as well as fetal conditions. The final model was validated using an external data set of the Consortium on Safe Labor after applying the same inclusion and exclusion criteria. We compared the area under the curve (AUC) of our predictive model and the simplified Bishop score. Results Of the 835 women, 563 (67%) had vaginal delivery. Factors associated with vaginal delivery included later gestational age at delivery, higher parity, more favorable simplified Bishop score, and preterm prelabor rupture of membranes. Factors including older maternal age, non-Hispanic Black race, higher body mass index, and abruption were associated with decreased likelihood of vaginal delivery. In the external validation cohort, 1,899 women were analyzed, of whom 1,417 (75%) had vaginal delivery. The AUCs of simplified Bishop score and the final model were 0.65 (95% CI 0.59-0.66) and 0.73 (95% CI 0.72-0.79), respectively, for the external validation cohort. The online calculator was created and is available at www.medstarapps.org/obstetricriskcalculator/ and in the Obstetric Risk Calculator mobile application in the Apple App Store and Google Play Store. Conclusion Our externally validated model was efficient in predicting vaginal delivery after preterm induction with unfavorable cervix.
