The pelvis, which is the bowl-shaped structure that makes up the hips, attaches the legs to the spine. The pelvis supports the spine, which supports the entire upper body. The back of the pelvis is the sacrum, one of the strongest bones in the body. During pregnancy, the ligaments soften and get a little longer, allowing the bones of the pelvis to relax to fit a baby through. The special hormone that causes this to happen is called relaxin!