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

Abstract

Debate continues over the respective merits of the supine and upright birthing positions, but there has been no accurate comparison of the effects of posture on the bony pelvic dimensions. The present study, enrolling 35 nonpregnant women 22 to 43 years of age, was an attempt to determine whether birthing position influences the dimensions of the pelvic outlet as viewed using a vertically open, low-field 0.5-T magnetic resonance imaging system and body flex surface coil. Twenty-five participants were nulliparous, and 10 were parous (9 with 1 child and 1 with 2 children). All the latter women had delivered at lest 9 months before the study. Images were recorded with women in the supine, hand-to-knee, and squatting positions. T1-weighted, fast spoiled gradient-echo sequences were recorded with the patient in the midsagittal, axial, and oblique (sacral promontory to the top of the symphysis) planes. Diagnostic-quality images were consistently obtained in this study. The sagittal outlet was wider in both the hand-to-knee and squatting positions than with the women supine. The same was the case for the interspinous diameter. The intertuberous diameter was greater with squatting than with the supine position. The only parameter that was significantly smaller in the upright squatting position than the supine position was the obstetric conjugate. Transverse diameters were essentially the same in all positions. Parous women were significantly older than the nulliparas and had slightly larger pelvic dimensions, but only the difference in sagittal outlet in the squatting position was significant. Whereas no parameters seemed to influence absolute pelvic measurements in supine women, the change in the obstetric conjugate on changing to a squat was greater in younger women. Taller women had a more marked increase in interspinous diameter in changing from the supine to the hand-to-knee position. The same women also had a greater change in obstetric conjugate when changing from the supine to the hand-to-knee position. Postural differences may significantly increase pelvic dimensions in women of reproductive age. These findings provide a rationale for long-standing practices of changing birthing position to make vaginal birth easier.