How many vertebrae are fused into the sacrum?

The sacrum is typically made from five vertebrae that fuse during development into one triangular bone. It connects the spine to the hip bones at the sacroiliac joints and helps transfer body weight to the pelvis and legs. The fusion usually completes by early adulthood, though timing can vary. The sacrum also helps form the back wall of the pelvis and provides attachment points for muscles and ligaments. Its shape is important for standing balance and efficient walking.