Ultrasonographic predictors of successful labor induction and vaginal delivery within 24 h in nulliparous women undergoing induction before the onset of labor

Labor induction is widely used in term pregnancies; however, accurately predicting induction success remains challenging, particularly in nulliparous women with unfavorable cervical conditions. This study aimed to evaluate the predictive performance of ultrasonographic parameters and the Bishop score for successful labor induction and vaginal delivery within 24 h. This prospective observational study included term nulliparous women with a Bishop score ≤ 6 undergoing labor induction before the onset of labor. Pre-induction ultrasonographic assessment included cervical length, head–perineal distance, head–external os distance, and cervical angles. Receiver operating characteristic (ROC) analysis and multivariable logistic regression were used to identify predictors of induction success and vaginal delivery within 24 h. Of the 141 women included in the final analysis, 108 (76.6%) achieved vaginal delivery, while 33 (23.4%) underwent cesarean delivery due to failed induction. Women who achieved vaginal delivery had a significantly shorter median cervical length (27 mm IQR 24–31 vs. 35 mm IQR 31–39), shorter head–external os distance (33 mm IQR 30–36 vs. 40 mm IQR 36–44), and shorter head–perineal distance (47 mm IQR 44–50 vs. 53 mm IQR 49–57) compared with those who required cesarean delivery (all p < 0.001). On multivariable logistic regression analysis, cervical length (OR 0.85, 95% CI 0.75–0.96; p = 0.01) and head–perineal distance (OR 0.93, 95% CI 0.88–0.98; p = 0.01) remained independently associated with induction success. Among women who achieved vaginal delivery, 85 (78.7%) delivered within 24 hours. Cervical length, head–external os distance, and the Bishop score showed modest discriminative ability for predicting delivery within 24 hours on ROC analysis; however, none of these parameters, including the Bishop score, remained independently associated with this outcome in multivariable analysis. In term nulliparous women with unfavorable cervical conditions, pre-induction ultrasonographic assessment—particularly cervical length and head–perineal distance—provides more objective and superior prognostic information for predicting induction success compared with the Bishop score. However, no independent predictors of vaginal delivery within 24 hours were identified.