The bony pelvis is engineered for stability, bearing the weight of the upper body and providing attachment for powerful leg muscles. However, this rigid structure must temporarily transform into a flexible passage to accommodate the birth of a baby. This physiological adaptation involves a series of hormonal changes and precise mechanical movements that allow the pelvis to expand.
The Pelvic Architecture
The pelvis is not a single bone but a complex girdle formed by the fusion of five distinct bones. It includes the two large hip bones—each composed of the ilium, ischium, and pubis—which connect to the sacrum and the small tailbone, the coccyx. These bones are held together by specialized joints that are normally quite firm, allowing very little movement.
The two main joints involved in childbirth are the pubic symphysis and the two sacroiliac joints. The pubic symphysis is a cartilaginous joint connecting the left and right pubic bones at the front. The sacroiliac joints link the hip bones to the sacrum, and these joints provide the stability needed for walking and standing.
Hormonal Preparation for Expansion
The peptide hormone relaxin, produced by the ovaries and the placenta, triggers this skeletal transformation. Relaxin acts directly on the connective tissues, initiating the softening and stretching of the ligaments and cartilage that stabilize the pelvic joints. This preparation begins early in pregnancy, long before labor contractions start.
The hormone’s effect is to increase the elasticity of the pubic symphysis and the sacroiliac ligaments. This joint laxity is a prerequisite for the widening and shifting of the bones required to create a larger birth canal.
The Dynamic Movements During Labor
As labor progresses and the baby begins to descend, the softened pelvic joints undergo dynamic movements to enlarge the passage. This process involves changes at both the pelvic inlet (the top opening) and the pelvic outlet (the bottom opening). The baby’s head acts as a wedge, facilitating these shifts as it navigates the curving path of the birth canal.
The pubic symphysis can widen by several millimeters, increasing the distance between the pubic bones. This small increase at the front translates into a significant gain in the overall diameter of the pelvis. Simultaneously, the large, triangular sacrum tilts and moves in a process called nutation and counternutation, which alters the dimensions of the pelvic cavity.
The movement of the sacrum is crucial for two different stages of labor. To open the pelvic inlet for the baby to enter, the sacrum subtly tilts backward, which is often facilitated by a posterior pelvic tilt of the mother. Conversely, to open the pelvic outlet for the final push, the sacrum changes its position, and the coccyx moves backward. This increases the anteroposterior diameter of the outlet, providing the final space needed for the baby’s head to pass through.
Postpartum Stabilization and Healing
After delivery, the body begins stabilizing the mobile joints. Relaxin levels gradually decrease, allowing the ligaments and cartilage to begin tightening again. This restoration of stability is a slow process that occurs over several weeks to months.
During this healing period, the lingering joint laxity can lead to common discomforts like pelvic girdle pain (PGP) or symphysis pubis dysfunction (SPD). These conditions arise because the pelvic joints remain unstable while the ligaments regain their strength. Full stabilization is a gradual process, and while most discomfort resolves within the first few months, some individuals may experience residual pain for a longer time.