OBJECTIVE The aim of our study was to measure the impact of supine and upright birthing positions on MR pelvimetric dimensions. MATERIALS AND METHODS MR pelvimetry was performed in 35 nonpregnant female volunteers in an open 0.5-T MR imaging system with patients in the supine, hand-to-knee, and squatting positions. The obstetric conjugate; sagittal outlet; and interspinous, intertuberous, and transverse diameters were compared among positions. RESULTS With patients in the hand-to-knee and squatting positions, the sagittal outlet (11.8 +/- 1.3 cm and 11.7 +/- 1.3 cm) exceeded that in the supine position (11.5 +/- 1.3 cm; p = 0.002 and p = 0.01, respectively), as did the interspinous diameter (11.6 +/- 1.1 cm and 11.7 +/- 1.0 cm vs 11.0 +/- 0.7 cm; p < 0.0001, in both cases). Intertuberous diameter was wider with patients in the squatting position than in the supine position (12.7 +/- 0.8 cm vs 12.4 +/- 1.1 cm; p = 0.01). Only the obstetric conjugate was smaller with patients in the upright squatting position than in the supine position (12.3 +/- 0.8 cm vs 12.4 +/- 0.9 cm; p = 0.01). Transverse diameter did not change significantly in any position. CONCLUSION An upright birthing position significantly expands female pelvic bony dimensions, suggesting facilitation of labor and delivery.