Measurement of Pubic Symphysis Width in Different Birthing Positions Using Ultrasound

Abstract

Studies have shown that certain postures, such as squatting and standing, may facilitate labour and birth and may be used by some practitioners. However, a rigorous biomechanical understanding of how different positions affect a woman’s pelvis during labour is currently lacking. It was hypothesized that there is a difference in relative pelvic bone position and orientation between different birthing positions that results in changes in pubic symphysis width. Therefore, we evaluated pubic symphysis width in positions that are used in clinics like lithotomy position and supine, and those suggested by midwives, such as standing and squatting, through three-dimensional (3D) reconstruction of pubic symphysis ultrasound images. Eleven women in their seventh to ninth month of pregnancy were recruited to be participants. A 3D freehand ultrasound system was used to scan the pubic symphysis of each participant in the four birthing positions. Image and position data of the probe were acquired. In each position, images were resliced from the reconstructed 3D volume and the same landmarks at the superior and inferior parts of pubic symphysis were located. The results were consistent with two comparable studies. The average superior and inferior pubic symphysis widths in lithotomy, squatting and standing positions were larger than those in the supine position by approximately 1mm. However, there was no significant difference between lithotomy, squatting and standing positions. The angles between right and left bones were calculated in an anatomical coordinate system. A significant difference was only found between standing and squatting and lithotomy in the rotation angle about the medio-lateral axis. The results suggest that positions with thigh hyperflexion and gravity effect make a difference in pubic symphysis width and might have a positive effect in childbirth. However, due to the small sample size, no definite conclusion can be drawn.