During pregnancy, the hormone relaxin turns stiff ligaments into stretchy bands throughout the entire body.

Relaxin is a polypeptide hormone primarily produced by the corpus luteum and the placenta during pregnancy. Its concentration peaks during the first trimester and again during delivery to facilitate the softening of the cervix and the relaxation of pelvic ligaments. One of its most critical roles is widening the pubic symphysis, a cartilaginous joint that typically moves very little but can expand significantly to allow the fetus to pass through the birth canal.Research published in the American Journal of Epidemiology indicates that the systemic nature of relaxin means it does not just target the pelvis. It affects collagen remodeling throughout the musculoskeletal system, which reduces the structural integrity of the longitudinal arches in the feet. A study by the University of Iowa followed pregnant women and found that about 60 to 70 percent experienced wider and longer feet, with arch height decreasing significantly during the first pregnancy.The hormone works by inhibiting collagen synthesis and increasing the production of collagenase, an enzyme that breaks down collagen fibers. This biochemical shift increases tissue elasticity but also makes joints more prone to injury and instability. While relaxin levels drop sharply after childbirth, the structural changes to the bones and ligaments in the feet often do not revert to their pre-pregnancy state, leading to a permanent increase in shoe size for many women.