“First comes the stretching
In order to squeeze something the size of a watermelon through something the size of a lemon, a woman’s body must stretch from the inside out—which, like a comic-book hero, it manages to do thanks to a hurricane of hormones.
Starting near the end of a woman’s second trimester, the body releases a hormone called relaxin, which causes the joints in her pelvis to literally relax, explains Dr. Stephanie Romero, an OBGYN and assistant professor at the University of South Florida. The body also releases the hormone progesterone during pregnancy, which (among many things) loosens a woman’s ligaments and muscles. This looseness helps the bones in the pelvis expand during labor, and thus accommodate the baby’s head as it works its way down the birth canal.”