The sacrum is a single wedge-shaped bone formed by the fusion of five sacral vertebrae. In childhood, the sacral vertebrae are separated by intervertebral discs in much the same manner as the upper parts of the spinal column. However, around the age of 20 these vertebrae start to fuse by ossification of the discs. This is a gradual process and it is not uncommon for ossification to only be completed up to or even beyond 40 years. The sacrum is wider in proportion to the length in females although the first sacral vertebra (S1) is larger in males.