Movements of the pelvic bones of expectant mothers during vaginal delivery.

Subtle but demonstrable movements in the expectant mother's pelvis occur during vaginal delivery in all the pelvic joints and anatomical planes of the body (sagittal, frontal, and transverse). The purpose of these movements is to gradually expand the space in the lesser pelvis via widening of the individual pelvic planes so that the newborn's head can enter the pelvic inlet, safely pass through the narrow planes of the pelvis, and through the pelvic outlet. From the point of view of biomechanics, these movements are described in literature as counternutation and nutation of the sacrum and iliac bone. The counternutation of the sacrum helps to expand the plane of the pelvic inlet. The nutation of the sacrum assists in expanding the plane of the pelvic width, height, and outlet. These physiological movements are affected by the body constitution, the state of the myofascial and skeletal systems of the mother, and furthermore, by hormonal disjunction of the connections in the expectant mother's pelvis together with the progress of the delivery mechanism itself. The main factor that determines the range of movement in the individual joints, and therefore adequate expansion of the individual pelvic planes, is the position of the mother during delivery. Engagement of active movements of the mother together with application of passive stretching of the soft tissues in the lower lumbar area and in the hip joints are both needed for maximum expansion of the individual pelvic planes and utilization of the maximum useful capacity of the mother's pelvis during delivery. These movements help invoke the abduction forces on muscles, tendons, and ligaments in the pelvis that lead to the optimum setting of the joints during which delivery movements happen. The specific movements in the pelvic joints predetermine whether nutation or counternutation is possible, and therefore if the newborn's head can progress to the pelvic inlet or pass through the narrow and wide pelvic planes, and the pelvic outlet. The knowledge of these biomechanical principles and movements in the pelvis during delivery enables obstetricians and midwives to understand how the movements in the hip joints of the expectant mother can positively impact the spatial ratios in the lesser pelvis, and how to support further progress in the event of non-progressive labour.