Balancing anatomical limits and dynamic adaptation: Understanding the determinants of successful vaginal breech...

To evaluate the association between obstetric parameters, MRI pelvimetry findings, and delivery outcomes in term singleton breech pregnancies attempting vaginal birth.

This retrospective observational study included 80 women with singleton breech presentations who underwent MRI pelvimetry and attempted vaginal delivery at Hannover Medical School between August 2021 and December 2024. The primary outcome was the mode of delivery (vaginal vs. intrapartum cesarean). Secondary analyses explored associations between obstetric parameters, neonatal outcomes, and the need for obstetric maneuvers. Statistical analyses included univariable logistic regression and receiver operating characteristic (ROC) curve assessment, to provide descriptive context for the observed univariable associations.

Of 80 cases, 57 (71.3%) achieved vaginal delivery and 23 (28.7%) required intrapartum cesarean section. Women with vaginal delivery had significantly larger pelvimetric dimensions, including the obstetric conjugate (12.9 vs. 12.4 cm, p = 0.009) and pubic angle (95.8° vs. 91.6°, p = 0.020). In descriptive ROC analyses, intrapartum course variables showed higher discriminatory values within the study sample. Prolonged oxytocin augmentation (unadjusted p = 0.009; AUC = 0.80; BH-adjusted p = 0.204) and extended active first stage of labor (unadjusted p < 0.001; AUC = 0.77; BH-adjusted p = 0.026) were associated with cesarean delivery; however, after adjustment for multiple testing, only the duration of the active first stage remained statistically significant. No association was found between birth weight and head circumference -whether estimated by ultrasound or measured after birth-, or fetal leg position and successful vaginal delivery. In the vaginal delivery group, adverse neonatal outcomes correlated with longer rupture of membranes-to-birth intervals and prolonged first and second stages of labor (p < 0.05), whereas pelvimetric measures showed no significant association.

In term breech deliveries, intrapartum dynamics showed stronger associations with intrapartum cesarean and neonatal outcomes than MRI pelvimetric dimensions, whereas fetal anthropometric parameters and leg position did not show comparable associations. MRI pelvimetry offers anatomical information at baseline, while continuous intrapartum evaluation reflects dynamic processes that are more closely associated with observed outcomes in vaginal breech birth.