MRI visualization of maternal pelvic and perineal anatomical changes during the second stage of labor: a feasibility study

OBJECTIVE: Maternal pelvic anatomical changes during the second stage of labor (SSL) remain poorly understood because of the difficulty in visualizing internal structures in vivo. Mechanical models have attempted to predict these changes but lack real-time confirmation. This proof-of-concept study assessed the feasibility of using open-field magnetic resonance imaging (MRI) to visualize maternal and fetal structural movements during the SSL. MATERIALS AND METHODS: Twenty-seven term pregnant women were recruited for an open-field MRI study at a single center. Each participant was scheduled for scans in a 1-T open magnet within 2 h before labor onset and again during the SSL. Owing to logistical constraints, seven patients underwent both pre-labor and intrapartum MRI. Qualitative assessments of intrapartum pelvic anatomy changes were performed and supplemented by basic quantitative measurements of pelvic dimensions. RESULTS: Intrapartum MRI was successfully performed in seven women (aged 23-34 years). All showed marked perineal expansion and deformation of the pelvic floor during fetal descent. The iliococcygeal portion of the levator ani flattened and reversed its curvature under fetal head pressure, corresponding to the perineal bulge. Posterior coccygeal deflection occurred in all the patients (mean coccyx-symphysis increase of ~ 0.8 cm), along with vaginal canal distension. Six patients had a partially filled bladder, which was displaced cranially as the fetal head descended. No adverse events occurred during MRI acquisition. CONCLUSION: Open-field MRI during the SSL is technically feasible and provides novel real-time observations of pelvic floor adaptations to fetal descent. These findings improve the understanding of intrapartum pelvic mechanics but require confirmation in larger cohorts. RELEVANCE STATEMENT: Open MRI offers a way to observe directly how maternal pelvic structures adapt during childbirth in real time. This feasibility study's insights are descriptive and hypothesis-generating, laying the groundwork for future research on protecting the pelvic floor during delivery rather than providing immediate clinical recommendations. By clarifying how maternal pelvic structures adapt to fetal descent, these MRI findings can inform strategies to minimize perineal trauma, guide interventions (e.g., episiotomy, not emptying the bladder before maternal efforts), and ultimately improve maternal postpartum pelvic floor health. KEY POINTS: Changes in the maternal pelvis during the second stage of labor remain poorly understood. Intrapartum open MRI can feasibly capture maternal pelvic anatomy in real time. Fetal descent markedly deforms the maternal pelvic floor (muscles, ligaments, and organs). Crowning is associated with significant anal sphincter stretching on MRI. Preliminary MRI observations may inform childbirth mechanics but are not yet generalizable.