The Pelvic Framework
The female pelvis forms a basin-like structure at the base of the spine, providing support and protecting internal organs. This bony ring is composed of several bones, including the sacrum and coccyx at the back, and the two large innominate bones, which form the sides and front. Each innominate bone is a fusion of three bones: the ilium, ischium, and pubis. These interconnected bones create the framework through which a baby must pass during birth.
While the bones themselves do not grow larger or “expand” during labor, the joints connecting them allow for subtle yet significant movement. The pubic symphysis, located at the front where the two pubic bones meet, is a cartilaginous joint with limited flexibility. At the back, the sacroiliac joints connect the sacrum to the iliac bones. These joints, along with the pubic symphysis, play a central role in the pelvis’s ability to adapt for childbirth.
Hormonal Preparation for Birth
The body begins preparing for childbirth weeks, or even months, before labor, largely influenced by a cascade of hormones. A primary hormone involved is relaxin, produced by the ovaries and placenta during pregnancy. Relaxin’s main function is to soften and relax the ligaments and cartilage throughout the body, particularly within the pelvis. This softening increases the elasticity and flexibility of the pubic symphysis and the sacroiliac joints.
This increased flexibility is not about making bones bigger but about allowing for greater mobility at the joints. By softening the connective tissues, relaxin facilitates the slight separation and movement of the pelvic bones during birth. Other hormones, such as estrogen and progesterone, also contribute to changes in connective tissues during pregnancy. Relaxin has the most direct and pronounced effect on the pelvic ligaments and cartilage, priming the pelvis for the dynamic movements of labor.
Pelvic Adaptation During Labor
During labor, the softened pelvic joints and ligaments allow the bones to undergo subtle yet highly coordinated movements, altering the dimensions of the birth canal. As the baby descends, the angles and relationships between the sacrum, coccyx, and innominate bones can shift. For instance, the sacrum can tilt backward and the coccyx can move out of the way, which helps to increase the space at the pelvic outlet.
The pubic symphysis, made more pliable by relaxin, can separate by a few millimeters, further widening the front of the pelvis. This slight separation, along with movements at the sacroiliac joints, allows the pelvic inlet and outlet to subtly change shape. These changes are not a forceful splitting but rather a gentle yielding and repositioning that optimizes the passage for the baby’s head and body. The baby’s own movements, such as molding of the head and rotation, also interact with these pelvic adaptations to facilitate passage.