How to open the pelvis for birth
The pelvis has three major components: the sacrum, the coccyx, and the two os coxae, which consist of the ischium, the ilium, and the pubis (aka the two halves of your pelvis). If you’re preparing for labor, understanding how to open the pelvis for birth starts with knowing how this structure moves.
The pelvis has three articulations, or points of movement: the sacrum and coccyx at the sacrococcygeal symphysis (tail bone); the sacrum and ilium at the sacroiliac joints (the back of the pelvis); and the pubic bodies at the pubic symphysis (the front of the pelvis). These three articulations allow for greater displacement and mobility of the pelvis during pregnancy to aid in the facilitation of fetal descent through the pelvis for childbirth (Hemmerich et al, 2019). The pubic symphysis increases in width and mobility during pregnancy, reported to be around 3 mm of an increase in width (Becker et al, 2010). However, this increase could be as much as 20 mm wider, which may be related to the number pregnancy (Becker et al, 2010). This increase in width can begin in the first trimester, and gradually increases throughout the duration of pregnancy (Becker et al, 2010).
The acetabulum is the articulation between the femur and the pelvic portions of the ilium, ischium, and pubis (Lewis et al, 2017). This articulation is a synovial ball-and-socket joint, which allows for motion in the sagittal, frontal, and transverse plane (Lewis et al, 2017).
The pelvis rotates in three cardinal axes: the mediolateral axis, the anteroposterior axis, and the vertical axis (Lewis et al, 2017). The mediolateral axis rotation results in an anterior or posterior pelvic tilt, which influences the relative location of the sacrum and sacral promontory. The anteroposterior axis rotation results in pelvic drop or hikes. The vertical axis rotation results in pelvic rotation, where one hip is anterior to the other hip.
Opening each level of the pelvis may happen intuitively for the birthing person, where they will move in a way subconsciously to open their pelvis more to allow for fetal descent (Davis et al, 2012). Deliberately opening the pelvis during labor will require knowing what the fetal station, or location of the fetal head, is within the pelvis, as the desire would be to create more space in the level that the baby is trying to descend though.
Fetal station is determined by the location of the baby’s head in relation to the ischial spines. 0 station indicates that presenting portion of the fetal head is aligned with the ischial spines. Minus numbers indicate that the fetal head is above the ischial spines, and not yet engaged into the pelvis. Plus numbers indicate that the fetal head is below the ischial spines (Davis et al, 2012).
Fetal positions are determined by the fetal occiput, or posterior portion of the skull, in reference to the maternal pelvis. The positions can be occiput posterior, in which the fetal back is directly aligned with the maternal spine; occiput anterior, in which the fetal back is aligned with the maternal abdomen; occiput transverse, in which the fetal back is aligned with either the left or right maternal hip; and then towards the right or left in any of those previously listed positions.
The Top of the Pelvis: The Inlet
The inlet of the pelvis comprises of the pelvic brim, pubic bone, and sacral promontory. The inlet of the pelvis is measured by the transverse diameter, the widest distance between the left and right side of the pelvic brim, and the obstetric conjugate, measured from the pubic symphysis to the sacral promontory (Pattinson et al, 2017). The widest diameter of the pelvic inlet is the transverse diameter, therefore the fetal head wants to descend through this portion of the pelvis in an occiput transverse position in order to align the widest diameter of their head, and eventually shoulders, to the inlet.
The pelvic inlet diameter can be opened with external rotation of the femurs, that widens the transverse diameter, and a posterior pelvic tilt that widens the obstetric conjugate (Lee et al, 2004). Movements such as deep, wide knee squats with a posterior pelvic tilt could be an ideal labor movement to help the baby first engage into the inlet of the pelvis.
Opening the Inlet
The Midpelvis
The midpelvis pelvic level comprises of the center of the pubic bone, the ischial spines, the sacrococcygeal symphysis, and the pelvic floor (Pattinson et al, 2017). The ischial spines are less prominent in the female pelvis compared to male to facilitate fetal descent during childbirth (Lewis et al, 2017). The midpelvis is measured by the interspinous diameter, or the diameter between the ischial spines, and the anteroposterior diameter, or the diameter from the center of the pubic symphysis to the sacrococcygeal symphysis (Pattinson et al, 2017).
The midpelvis diameters can be opened with diagonal or uneven hip movements, which would increase the diameter of the interspinous diameter (Lee et al, 2004). In addition, sideways movements would be ideal as the baby descends through the midpelvis as the pelvic floor opens more side to side, as opposed to forward or backwards movements.
The widest diameter of the midpelvis is diagonal from the pubic bone to between the ischial spines and sacrum on the contralateral side. Therefore, the fetal head wants to descend through this portion of the pelvis diagonally, in either a left occiput anterior or right occiput anterior position. At the bottom of the midpelvis, the fetal head needs to rotate to descend through the pelvic floor, in which opens more anterior to posterior, and therefore will rotate to an occiput anterior position to descend the pelvic floor.
Opening the Midpelvis
The Outlet
The outlet of the pelvis comprises of the pubic arch, ischial tuberosities, the sacrococcygeal symphysis, coccyx, and the pelvic floor (Pattinson et al, 2017). The outlet is measured by the intertuberous diameter, or the diameter between the ischial tuberosities, and the anteroposterior diameter, the diameter from the center of the pubic symphysis to the sacrococcygeal symphysis (Pattinson et al, 2017). The widest diameter of the outlet is the anteroposterior diameter, as the sacrum and coccyx can move backwards to create more space during pushing. Therefore, the fetal head wants to rotate to an occiput anterior position in order to move underneath the pubic arch and descend through the outlet of the pelvis and pelvic floor.
The outlet’s diameters can be increased with an internal rotation of the femur, that widens the intertuberous diameter (Lee et al, 2004). The outlet’s anteroposterior diameter can be widened with either an anterior pelvic tilt to aid in the sacrum and coccyx moving posteriorly (Lee et al, 2004), or with maintaining space for the sacrum to move during fetal descent, or by avoiding supine positions during pushing.