Background <p>Conversion from epidural labor analgesia (ELA) to epidural surgical anesthesia (ESA) is a common clinical procedure in obstetric practice. However, its failure rate remains unacceptably high (15%–38% in previous reports), posing substantial risks to maternal and neonatal safety, including increased reliance on general anesthesia and associated adverse outcomes. This study aimed to evaluate the impact of preanesthetic ELA interruption on the conversion efficacy of prolonged ELA (≥ 8&#xa0;h) to ESA and to explore underlying anatomical and physiological mechanisms using lumbar magnetic resonance imaging (MRI).</p> Methods <p>This integrated study comprised two complementary components: a prospective cohort study and a controlled before-after MRI study. For the prospective cohort study, 167 parturients requiring conversion from prolonged ELA (≥ 8&#xa0;h) to ESA were assigned to either Group S (ELA interrupted ≥ 30&#xa0;min before ESA initiation, <i>n</i> = 77) or Group NS (ELA continued until ESA initiation, <i>n</i> = 88) based on clinical practice preferences and patient-related factors. The primary outcome was the ESA failure rate; secondary outcomes included intraoperative analgesic efficacy (visceral and incision pain visual analog scale [VAS] scores), muscle relaxation quality, and maternal satisfaction. Multivariable logistic regression was used to adjust for potential confounding factors at intervention assignment. For the MRI study, 11 parturients who underwent vaginal delivery with ELA underwent lumbar MRI scans in the supine position with left uterine displacement at 2&#xa0;h post-delivery (ELA active phase) and 24&#xa0;h post-delivery (ELA interruption phase). Key measurements included dural sac (DS) area, epidural space (ES) area, and apparent diffusion coefficient (ADC) values of spinal ganglia at the L1-L5 levels. All MRI volumetric quantifications were performed by two experienced radiologists using a standardized segmentation protocol to improve reliability.</p> Results <p>In the cohort study, Group S showed a significantly lower ESA failure rate compared with Group NS (13.0% vs. 29.5%, <i>P</i> = 0.014). Group S also had better intraoperative pain control, muscle relaxation, and maternal satisfaction. Multivariable logistic regression adjusted for maternal weight and ELA duration confirmed that longer ELA interruption time was independently associated with reduced odds of ESA failure (OR = 0.986, 95% CI 0.977–0.996, <i>P</i> = 0.006). The MRI study revealed that at 2&#xa0;h post-delivery, compared with 24&#xa0;h post-delivery, the DS area was significantly reduced and the ES area was significantly enlarged at all lumbar levels. Epidural fluid accumulation was observed in 18.2% of parturients at 2&#xa0;h. No significant differences in spinal ganglion ADC values were detected, indicating no neural edema.</p> Conclusions <p>Preanesthetic interruption of ELA for ≥ 30&#xa0;min significantly improves the conversion efficacy of prolonged ELA to ESA, as shown by reduced failure rates, enhanced anesthesia quality, and improved maternal outcomes. Mechanistically, ELA-related epidural space dilation, dural sac compression, and residual low-concentration local anesthetics (possibly associated with epidural fluid accumulation) may contribute to conversion failure, and these adverse changes can be relieved by preanesthetic ELA interruption.</p> Trial registration <p>ChiCTR2100047772 (prospective cohort study, registered in the Chinese Clinical Trial Registry on June 26, 2021; ChiCTR2200059311 (MRI study, registered in the Chinese Clinical Trial Registry on April 28, 2022).</p>