The question of whether hips “grow” during pregnancy is common, but the answer requires a biological distinction. The bones themselves, which make up the hip structure, do not grow or change size during the nine months of gestation. Instead, the temporary widening that many women experience is the result of joints and ligaments becoming significantly softer and more mobile. This change is a natural, temporary adaptation to prepare for childbirth.
Hormones That Cause Pelvic Mobility
The primary trigger for the softening of connective tissues is the hormone relaxin. Produced early by the corpus luteum and later by the placenta, relaxin’s main function is to prepare the reproductive system and the pelvis for delivery. Relaxin targets collagen, causing tissues to become more elastic and pliable. This increase in joint laxity begins to be noticeable around the second trimester and continues through pregnancy.
Progesterone works alongside relaxin to enhance this effect. It relaxes smooth muscle and connective tissue, synergistically supporting the softening process. The sustained presence of both hormones ensures the pelvic joints remain flexible. This hormonal shift facilitates the passage of the baby through the birth canal.
Anatomical Changes to the Pelvic Girdle
The pelvic girdle is a ring of bones, including the ilium, ischium, and pubis, connected by joints and strong ligaments. Widening is the slight separation of two key joints: the pubic symphysis and the sacroiliac (SI) joints. The pubic symphysis is a cartilaginous joint at the front of the pelvis with limited movement. Hormonal influence softens the cartilage and ligaments, allowing the pubic bones to separate slightly and increase the overall pelvic diameter.
The sacroiliac joints connect the sacrum, the triangular bone at the base of the spine, to the two large hip bones. Ligaments loosen significantly during pregnancy, increasing their mobility. This laxity allows the pelvis to “splay” a small amount, which is essential to create a wider exit for the baby during labor. This joint separation is the source of the temporary increase in hip width.
The increased mobility and instability in these joints often lead to discomfort, known as pelvic girdle pain (PGP). Mechanical forces change as the center of gravity shifts with the growing uterus, placing additional stress on the loosened joints. This combination of hormonal softening and biomechanical strain accounts for the change in posture and the feeling of a wider hip structure.
Postpartum Reversal of Hip Widening
The temporary nature of the hip widening is linked to the rapid decline in pregnancy hormones following delivery. Once the placenta is delivered, relaxin levels drop, initiating the process of joint and ligament tightening. This re-stabilization allows the pelvis to gradually return to its pre-pregnancy alignment, though the timeline varies.
Most women observe a substantial reduction in widening within the first few weeks to months postpartum. Joints and ligaments continue to tighten over several months, with the most significant changes seen within six to twelve months after birth. Studies indicate that the hips return substantially to their original size, often reversing about 70-80% of the initial widening.
A small, subtle change in hip width may persist for some women, typically measured as only one to two centimeters wider than pre-pregnancy. This minor change is not due to bone growth but rather a slight, lasting shift in pelvic alignment or residual laxity in the connective tissues. Postpartum recovery focuses on strengthening the surrounding core and pelvic muscles to restore stability and function.