the pelvic girdle forms a continuous ring, which surrounds the pelvic canal. This is specially adapted to the needs of childbirth in the female. This reproductive adaptation affects particularly the lesser pelvis; but the changes in this, affect the proportions and dimensions of the greater pelvis to a variable degree in the female.
Nature has designed the pelvis so that its joints become increasingly mobile during pregnancy, thanks to the softening effects of hormones on the ligaments. These ligaments are bands of tough fibrous tissue, which hold the bones of the pelvis together at the joints. These interweave and run in all directions and, with the help of the pelvic muscles, bind the whole pelvis into a single unit.
When the pelvic bones soften, they allow expansion of the pelvic joints, and this widens the diameter of the pelvic canal, making more space for the baby. The pubic joint in front is usually held firmly together but during the child birth it is capable of separating as much as half an inch to stretch the pelvic canal and not getting broken in the process of the childbirth. In addition, the joints between the sacrum and the hipbones, known as the sacroiliac joints, are able to make a slight pivotal movement forward and backward so as to allow the sacrum a certain degree of mobility such that the baby descends through the pelvic canal. It also makes it possible for the sacrum to tilt backwards so that the diameter of the pelvic outlet from front to back widens as the baby is emerging.
The tiny joint between the coccyx and the sacrum also becomes more mobile, enabling the coccyx to move back out of the way as the baby is being born.
In addition to this increase in pelvic mobility, the baby’s skull bones would not have been as yet fused as in an adult. Therefore they can also overlap against each other so as to allow a certain degree of ‘moulding’.