Mobility of the sacro-iliac joint
The surfaces of the sacrum and the hip bone have articular cartilage, and an articular cavity exists between them. Thus, although it is a joint in the narrow sense (synovial joint), the joint capsule of this joint is so tightly reinforced by the anterior sacro-iliac ligament, interosseous sacro-iliac ligament, and posterior sacro-iliac ligament that it can hardly be moved.
In cows and some other animals, progesteron act on these ligaments to make them more flexible, facilitating childbirth. This may also be the case in humans.