MR Obstetric Pelvimetry: Effect of Birthing Position on Pelvic Bony Dimensions

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.