The effect of maternal posture on pelvic outlet MR measurements

Introduction For centuries, there has been controversy around whether being upright or lying down has advantage for women delivering their babies. Different labor positions e.g. squatting, hand-to-knee position, sitting in birthing stools, are offered for parturition according to the cultural background and in the recent past even as “fashion” [1,2]. Changes in the pelvis under these conditions are anatomically not exactly known. Using conventional outlet views, it was shown that there was a significant increase in interspinal diameter in pregnant women in the last trimester on alteration of position from supine to sitting [3]. MR pelvimetry is widely accepted as the imaging modality of choice to assess the maternal bony pelvis in obstetrics [4,5]. With the advent of vertically open configuration magnet systems, patient s examination is not limited anymore to the supine position and examinations in the upright position were for example shown to be of value for assessment of the female pelvic floor as well as for MR defecography [6]. The aim of our study was to investigate if measurements of the pelvic outlet based on MR examinations performed in an open 0.5 T system are influenced by different labor positions. Methods MR pelvimetry was performed in 25 female non-pregnant volunteers (mean age 27 ± 4 y, mean weight 59 ± 7 kg, mean height 166 ± 5 cm) in an open 0.5 T MR system (Signa SP, GE, Milwaukee, WI) in supine, hand-to-knee (Figure 1: a) and squatting (Figure 1: b)positions using a T1-weighted GRE sequence (FSPGR, TR 150, TE 8.6, matrix 256x192, flip 60°, slthick. 5/0, 2 NEX) in the axial and midline-sagittal plane. Obstetric conjugate, transverse diameter, outlet sagittal diameter, transverse interspinal and intertuberous diameters were measured and results of the different positions were correlated.