Ultrasound Assessment of Breech Engagement: Breech Progression Angle and Prediction of External Cephalic Version Success

<b>Objectives</b>: To evaluate the role of breech progression angle (BPA), a novel transperineal ultrasound parameter, as a predictor of external cephalic version (ECV) success, and to compare BPA between breech and transverse lie presentations. <b>Methods</b>: This prospective exploratory study was nested within a randomized clinical trial (NCT06449430) at Virgen de la Arrixaca University Hospital, Murcia, Spain. Eligible participants were pregnant women ≥18 years with a singleton fetus in non-cephalic presentation at term, without contraindications to vaginal birth. BPA was measured transperineally following standardized methodology prior to ECV, performed under either spinal analgesia or propofol sedation. Logistic regression models adjusted for maternal and obstetric variables assessed the association between BPA and ECV success. Receiver operating characteristic (ROC) curves were generated to evaluate predictive accuracy. <b>Results</b>: A total of 117 women were included: 100 with breech and 17 with transverse lie presentations. Median BPA was significantly higher in breech compared with transverse lie (87.2° vs. 70.2°, <i>p</i> < 0.001). In the overall cohort, BPA was not significantly associated with ECV success (OR 0.97, 95% CI 0.94-1.00; <i>p</i> = 0.068). However, in breech presentations, BPA was independently associated with ECV success (adjusted OR 0.95, 95% CI 0.91-0.99; <i>p</i> = 0.015). The area under the ROC curve for BPA predicting ECV success in breech cases was 0.64 (95% CI 0.53-0.73). Predictive accuracy was poor for transverse lie (AUC 0.27, 95% CI 0.08-0.56). <b>Conclusions</b>: BPA measured by transperineal ultrasound does not provide clinically useful information for predicting the success of external cephalic version, either in breech or transverse lie.