During pregnancy, the busy placenta produces a hormone called relaxin that helps your whole body physically become a bit looser.
You need to seriously increase your flexibility down in your fetus's living quarters because, during pregnancy, your uterus grows from about the size of a tennis ball to the size of a beach ball. Relaxin relaxes the intrauterine ligaments, allowing the uterus and pelvis to expand. Relaxin also relaxes other parts of your body, like your arteries, which have to accommodate a much higher blood volume without sending your blood pressure through the roof. Your musculoskeletal system also loosens up, giving you more flexible joints and more curvature in the back to prepare for carrying and delivering a baby.