What needs to happen to have a successful vaginal birth?
Having a baby go through the pelvis is like putting a key through a keyhole
Having a baby go through the pelvis is like putting a key through a keyhole
With Optimal Fetal Positioning (OFP), the focus is on the position of the “key” - the key representing the baby. With Optimal Maternal Positioning (OMP), the focus is on creating space within the “keyhole” - the keyhole representing the pelvis. The baby goes through a range of movements within the pelvis through the labor and birth process and this range of movements is called “cardinal movements” - where a baby goes through a range of 7 movements from: engagement, descent, flexion, internal rotation, extension, external rotation and expulsion. Now we are going to explain where these cardinal movements take place within the various levels of the pelvis to see how your baby passes through your pelvis.
Greater & Lesser Pelvis
Within the pelvis itself, we have the “Greater Pelvis” and the “Lesser Pelvis”. The greater pelvis is where the baby resides during pregnancy, and when the baby reaches full-term pregnancy and is ready to start its journey through the ‘keyhole’, it will need the force of contractions to push it through the ‘lesser pelvis”.
Three levels of the Lesser Pelvis
Within the ‘lesser pelvis’, there are 3 levels of the pelvis - inlet pelvis (superior pelvis), mid-pelvis (middle pelvis) and outlet pelvis (inferior pelvis).
Within the pelvis itself, there are 2 landmarks that are essential in facilitating cardinal movements, that being the sacropromontory and the infamous pelvic floor.
When the pelvis opens at the inlet pelvis - this is called iliac and sacral contranutation.
When the pelvis opens at the outlet pelvis - this is called iliac and sacral nutation.
A full term baby needs the force of the contractions in order for baby to complete its cardinal movements through the pelvis. As long as this baby is enable to complete its cardinal movements, the mother will have a successful vaginal delivery. Hence, cardinal movements facilitate labor progress.
Greater Pelvis
One of the reasons why research shows that 50-60% of babies start labor in Left-Occiput-Tranverse (LOT) is because when babies are still ‘high’ - naturally so - the sacropromontory is in the way and the space available for baby is mostly LOT. Its only when baby drops deeper into the pelvis which happens spontaneously or when contractions start, that its head would have navigated around the sacropromontory to be able to get into Left-Occiput-Anterior (LOA). When in LOT, babies are working on engagement and descent.
Inlet Pelvis
For babies to make that 45 degree turn from LOT to LOA during labor, the force of the contractions will push the baby’s left parietal bone onto the sacropomontory to signal this baby to go asynclitic on one side, before going asynclitic on the other side - tipping its head to one side before tipping its head to the other side - to move into LOA where baby is now able to enter the inlet pelvis.
When babies are presenting in LOA at the inlet pelvis, it now has to chin tuck or flex its chin even more to get the occiput (the smallest diameter of the baby’s head) presenting at the widest diameter for the baby to enter the inlet pelvis.
Mid Pelvis
As the contractions continue, the second landmark within the pelvis is the pelvic floor. As the contractions enable this baby to internally rotate from LOA in the inlet pelvis to OA in mid-pelvis - an aligned pelvic floor is essential in ensuring that the baby does not go asynclitic (tip its head to one side which causes uneven pressure on the cervix and within the pelvic cavity) at this stage to cause labor to stall.
When labor stalls around 7cm dilation, a common cause is a misaligned pelvic floor that doesn’t give equal pressure on all sides that causes baby’s head to move from presenting occiput to asynclitic.
Assuming that labor is progressing well, the pressure of the aligned pelvic floor will facilitate this baby turning to OA at mid-pelvis, and at this stage, first time mothers would usually be anywhere between 6-10cm dilated.
Outlet Pelvis
As labor progresses, the contractions will enable this baby to move from OA at mid-pelvis to OA at outlet pelvis where baby is focusing on extending its head to be born, after which it will ‘restitute’ or externally rotate its shoulders to turn to the widest diameter of the outlet pelvis - front to back and ‘expulsion’ where baby is being born!
Optimising the Pelvis
It is essential to understand where each cardinal movements take place at which level of the pelvis so expectant parents themselves and their birth team will understand which part of the pelvis needs to open and create space for the baby depending on which stage of cardinal movements this baby is at. When a mother knows how to work with her body using maternal positioning otherwise known as OMP Active Birth Positions, maternal movements otherwise known as OMP Pelvic Mobility Protocols and OMP Comfort Measures, she is able to optimise her pelvis to create space for her baby at every stage of labor.