The perception that hips widen during pregnancy is driven by a natural physiological process that prepares the pelvis for childbirth. This change is not due to the hip bones growing, but rather the temporary softening and separation of the joints holding the pelvis together. This structural modification is a temporary, measurable change designed to increase the capacity of the birth canal.
The Hormonal Mechanism of Pelvic Change
The biological driver for pelvic softening is the hormone Relaxin, produced by the ovaries and the placenta. Relaxin targets dense connective tissues, remodeling collagen fibers within the pelvic ligaments and cartilage. Levels begin rising around the sixth week of pregnancy and peak near the end of the first trimester.
The main joints affected are the pubic symphysis at the front and the two sacroiliac joints at the back. Relaxin’s influence makes these normally rigid joints pliable and mobile. This joint laxity allows the pelvic structure to become less stable, creating necessary flexibility that continues throughout the pregnancy.
How Much Pelvic Joint Separation Occurs
The actual bony separation is minimal but measurable in millimeters, distinguishing it from the perceived widening of the entire hip structure. In a non-pregnant person, the pubic symphysis—the joint at the front—maintains a normal gap of 4 to 5 millimeters. Hormonal effects during pregnancy cause this gap to increase by an additional 2 to 3 millimeters.
Studies immediately postpartum show the average separation of the pubic symphysis is around 7.6 millimeters. This measurable change increases the space between the pubic bones, but does not widen the hip bones themselves. Perceived hip widening is usually a combination of this minor joint relaxation, outward pressure from an anterior pelvic tilt, and the deposition of fat tissue in the hip and thigh region.
A gap exceeding 10 millimeters is classified as Pubic Symphysis Diastasis, which can cause pain and instability. The extent of joint loosening varies considerably between individuals and is not determined solely by the size of the fetus or the mode of delivery. While increases up to 20 millimeters have been observed, this is uncommon.
The Function of Pelvic Widening in Delivery
The increased mobility provided by the softened pelvic joints serves the functional purpose of creating a more flexible birth canal. The normally rigid pelvis must adapt its shape to allow the baby to pass through during labor and delivery. This flexibility is achieved by the movement of the three main pelvic articulations, particularly the pubic symphysis and the sacroiliac joints.
The slight widening allows the sacrum, the triangular bone at the base of the spine, to tilt and rotate. This movement expands both the pelvic inlet at the top and the pelvic outlet at the bottom. By increasing the diameter of the pelvic space, the body facilitates the complex maneuvers the baby must perform to descend.
Postpartum Pelvic Recovery
The widening is not permanent, and the pelvis begins recovery immediately after delivery. As Relaxin clears from the bloodstream, the pelvic ligaments gradually begin tightening and regaining their pre-pregnancy rigidity. For most women, the physiological separation of the pubic symphysis returns to its non-pregnant width of 4 to 5 millimeters within about six weeks postpartum.
While the ligaments recover relatively quickly, associated pain and residual instability can linger. Pain often resolves within the first month, but a separated pubic symphysis may take three to eight months to fully stabilize. Occasionally, minor, permanent shifts in pelvic alignment may remain, influencing long-term joint comfort. However, the overall structure of the pelvis is designed to return to its original state.