Relative CPD
Cephalopelvic Disproportion (CPD) is when the baby is too large for the pelvis, or the pelvis is too small for the baby, to descend through during birth. This may be the diagnosis after a prolonged pause in labor, lack of engagement (baby entering the pelvis), and eventual cesarean birth.
Relative CPD: Is baby too big or your pelvis too small?? Maybe not!
But, how common is CPD, and could your pelvis really just be too small for vaginal birth?
We’ve all heard the stories of women who were told they would never have a vaginal birth, surprising everyone with a vaginal delivery with their biggest baby!
CPD is hard to accurately understand its prevalence, but some research supports 1-8% [1] of birth may be affected. One consideration is a more sedentary lifestyle as a contributing factor, as the pelvis functions include supporting walking upright and birth. If we limit the function of walking and general movement, we may find the pelvis develops differently or they are musculoskeletal restrictions that inhibit the mobility of the pelvis during birth.
Another observation is that certain sports or professions emphasis external rotation and an anterior pelvic tilt, such as dance or gymnastics, which can make creating space asymmetrically to help with baby’s rotation more challenging. The reason is that we need one-half of the pelvis to be able to find internal rotation, adduction, and a posterior pelvic tilt on one side to help create space for the baby to rotate.
If we are stuck in an anterior pelvic tilt, abduction, and external rotation, then space may be limited for the baby to rotate through the pelvis, and may appear “stuck.”
So, the actual pelvis may not be too small, but rather lifestyle choices may contribute to a lack of mobility.
But what about relative CPD?
If someone’s pelvis is not structurally too small, and their baby is average size, what could have prevented labor progress??
Relative CPD is when:
the baby presents larger due to head position; a chin tucked head position tends to present the smallest diameter, while the chin forward or extended presents much larger!
baby’s position doesn’t align with the diameter of the pelvis properly, causing them to not be able to fit and rotate through the pelvis.
Lack of or restricted movement in the mother; the pelvis opens with various movement patterns depending on the level, not moving at all inhibits the creation of more space for the baby.
Baby’s Head Position
How baby is presenting their head can make them present smaller or larger!
If the baby’s chin is tucked, they present the smallest diameter of their head to the birth canal. The bones of the skull mold most easily in this position!
However, if the baby’s chin is forward or even extended, they present much larger! The diameter of their head appears to be larger by multiple centimeters! This is huge!
Babys head position may be influenced by the shape of the uterus. The left and anterior side tend to be more curved, prompting baby to tuck their chin.
In addition, the pelvic floor tension can support the chin tuck. If the pelvic floor has very low tone, it may not support baby’s head tucking. This may be the case after an epidural where the overall muscle tone lowers due to relaxation.
On the other side of the spectrum, if the pelvic floor has really high tone, or uneven tone, it may cause baby’s head to tilt sideways.
Baby’s Position & Pelvic Level
Each level of the pelvis has a different shape and general movement pattern that creates more space. We break down pelvic biomechanics in more depth in our childbirth education courses.
We want to align the baby’s head with the shape of the pelvic levels.
For example, when trying to support engagement or a baby entering the pelvis, the baby’s head tends to be the widest front to back, and the inlet tends to be the widest side to side. So, we want babies to be LOT, or left occiput transverse when they engage into the pelvis to best fit into that space.
If the baby is not aligned to the space available, we may find they have more difficulty descending and rotating through the pelvis.
For example, if the baby is posterior or OP, they tend to present chin forward so their head has a larger diameter due to the uterine shape. Then, with this larger diameter, they are not aligned to the larger opening of the pelvis, which would be side to side. The Baby’s head would likely overlap the pubic bone, and not engage into the pelvis. This is not because the baby is too big or the pelvis is too small, but rather the presentation and alignment were not ideal.
Movement
The pelvic joints have significant movement capability during pregnancy and birth. When we move, our pelvis movements and different spaces open or close. When we do not move or restrict our movement, space tends to not change.
If the baby is trying to engage and is aligned w the pelvic inlet, we can create more space at that pelvic level to support the baby’s engagement. If the baby is trying to rotate through the pelvis, we can create more space in the mid pelvis to support the baby’s rotation.
If we do not move or limit our movement, we may find that our labor has a prolonged pause because the space needed is not being created to support our baby’s descent through the pelvis.
What to do?
CPD may not be a common diagnosis, but relative CPD, or the appearance of a too-large baby/too small pelvis, may have a large role in how birth progresses. If we focus on supporting the baby’s chin position to present smaller, baby’s fetal position to align with the pelvic level shapes, and encourage movement during labor, we may find that labor suddenly progresses when we may have thought otherwise!