The pelvis, a ring of bones composed of the ilium, ischium, pubis, and sacrum, functions as the central foundation for the spine and provides protection for internal organs like the bladder and uterus. Throughout pregnancy, this bony structure must undergo significant transformation to support the increasing weight of the developing fetus and, ultimately, to create a wider passage for birth. These necessary changes involve a complex interplay of hormones and physical adaptations that temporarily increase the pelvis’s flexibility and mobility.
Hormonal Triggers for Pelvic Mobility
Relaxin, a polypeptide hormone, is produced primarily by the corpus luteum in the ovary and later by the placenta. Relaxin’s main function is to prepare the body for childbirth by increasing the elasticity and flexibility of connective tissues throughout the body. While it affects various joints, its impact is most notable in the pelvic region. The hormone causes the collagen fibers within the ligaments to become more pliable, allowing the joints they stabilize to move more freely. Relaxin levels begin to rise almost immediately after conception, peaking in the first trimester, and then remaining elevated until delivery.
Widening of the Pelvic Joints
The increased laxity induced by Relaxin directly affects the three main joints of the pelvis. The pubic symphysis, the cartilaginous joint connecting the two halves of the pelvis at the front, is the site of the most noticeable movement. While typically having only 1 to 2 millimeters of movement, this joint can physiologically widen by an average of 7 to 8 millimeters during pregnancy to increase the pelvic outlet diameter. The two sacroiliac (SI) joints, located where the pelvis meets the base of the spine, also experience increased mobility and softening, allowing the sacrum to slightly move and rotate during labor. The combined expansion at the pubic symphysis and the SI joints increases the pelvic opening, which is essential for the baby’s passage, though this widening can sometimes lead to instability and pain, especially when the separation is greater than the typical range.
Changes to Supporting Muscles and Posture
The hormonal changes and structural shifts in the joints create secondary effects in the muscles that support the pelvis and spine. The pelvic floor must contend with both hormonal softening and the increasing downward weight of the growing fetus and uterus, which can cause the muscles to weaken or become overstretched, affecting their ability to maintain urinary and fecal continence. The physical weight of the growing baby shifts the center of gravity forward and upward, forcing the pregnant person to compensate with a change in posture. The natural curve of the lower back (lumbar lordosis) increases significantly to maintain balance, creating an exaggerated “swayback.” This postural change and the loosening of the SI joints place considerable strain on core stabilizing muscles, leading to common conditions like Symphysis Pubis Dysfunction (SPD) or sacroiliac joint pain. SPD is characterized by pain at the front of the pelvis, while SI joint pain is felt in the lower back or buttock area.
Postpartum Pelvic Recovery
Following delivery, the body begins the gradual process of reversing the structural and hormonal changes that occurred during pregnancy. The levels of Relaxin, which are responsible for the increased joint laxity, slowly begin to decrease, allowing the ligaments and joints to regain their pre-pregnancy firmness. This process of stabilization is not immediate, and the hormone can linger in the system for several months postpartum, especially if breastfeeding continues. The recovery of the pelvic floor muscles and connective tissue is also a slow process, with full tissue strength taking six to twelve months or longer to return. The joints and ligaments gradually tighten, and the body’s center of gravity shifts back, but patience and focused recovery work are often needed to fully regain pelvic stability.