Impact of supine vs. hip- and knee-flexed position on anteroposterior diameter and inclination of the female pelvic outlet: an MRI-based 3D reconstruction study.

Background: Maternal positioning influences pelvic biomechanics during labor; however, the anatomical mechanisms underlying position-related pelvic morphological changes remain incompletely understood. This study aimed to quantitatively evaluate pelvic morphological changes associated with a hip- and knee-flexed position compared with the supine position using three-dimensional (3D) reconstruction. Methods: In total, 27 nulliparous non-pregnant women underwent pelvic MRI examinations in both the supine and hip- and knee-flexed positions. A radiation-free MRI-based 3D reconstruction approach was used to measure pelvic diameters and angles. A total of 12 pelvic parameters were evaluated, including the pelvic inlet, midpelvis, outlet, external pelvic measurements, and pelvic inclination. Data normality was assessed before paired-samples t-tests were performed, and intraclass correlation coefficients were calculated to assess measurement reliability. Results: Compared with the supine position, the hip- and knee-flexed position significantly increased the anteroposterior diameter of the pelvic outlet (mean increase, 4 mm; Conclusion: A hip- and knee-flexed position is associated with specific and quantifiable changes in pelvic outlet morphology and pelvic orientation in non-pregnant women. These findings provide preliminary anatomical and biomechanical evidence regarding position-related pelvic mobility. Further studies in pregnant populations are required to determine the clinical relevance of these positional changes during labor.