The spinal cord
The human spinal cord and the vertebral column initially grow at the same rate during embryonic development. In this way, spinal segments and the vertebral bones for which they are named are aligned. Therefore, the spinal nerves emerge from the vertebral column at the same level as the spinal cord segment from which they arise. However, after the third month of gestation, each vertebral bone becomes larger compared to the associated spinal segment; therefore, the vertebral column grows approximately 25 cm longer than the spinal cord. (This explains why the spinal cord extends only as far as the upper lumbar vertebrae.) As a result, the spinal cord segment from which each pair of spinal nerves arises is no longer aligned with its associated vertebral bone.