relaxin

Relaxin is, true to its name, the hormone that loosens things up for childbirth. Imagine the body gently softening its own hinges and ligaments so the pelvis can open and the cervix can yield when the time comes. It is part of why some pregnant people feel their joints become looser and less stable.

It is a peptide hormone in the insulin family, produced mainly by the corpus luteum (and in pregnancy also by the placenta and the lining of the uterus). Its actions include softening the connective tissue of the pelvic ligaments and the cervix, helping remodel the uterus, and contributing to relaxation of blood vessels, which supports the increased blood flow that pregnancy demands.

Outside of obvious pregnancy effects, relaxin influences the cardiovascular and renal adaptations of pregnancy, helping the maternal circulation expand to meet the needs of the placenta and fetus.

An honest caveat: although relaxin clearly softens connective tissue, its precise role in human labor is less dramatic than the name suggests, and the long-held idea that it directly causes most pregnancy joint pain is more nuanced than once believed; individual responses vary considerably.