Mobility of the sacro-iliac joint
The auricular surfaces of the sacrum and the hip bone have cartilages, and an articular cavity exists between them. Thus, although it is a joint in the narrow sense (synovial joints), the sacro-iliac joint's joint capsule is so tightly reinforced by the ventral sacro-iliac ligament, interosseous sacro-iliac ligament, and dorsal sacro-iliac ligament that it can hardly be moved.
In cows and other animals, progesterone act on these ligaments to make them more flexible and facilitate childbirth. This may also be the case in humans.