Ascent of the conus medullaris from opposite the embryonic coccygeal spinal segments with nerve roots exiting directly opposite their vertebral levels starts at POD 27 (7, 8, 9).
- Thinning of caudal neural tube: Between POD 27 and 54, the caudal end of the newly formed neural tube undergoes retrogressive differentiation during which it becomes thinner, is less well developed, and eventually contains no marginal zone and only a rudimentary mantle zone. This thinning of the caudal neural tube makes it appear that the tip of the conus is gradually ascending relative to the adjacent vertebral column.
- Relatively faster growth of the spine: Beyond POD 54 retrogressive differentiation has ended, and the ascent of the conus is caused solely by a disparity in the growth of the vertebral column relative to the spinal cord such that the conus medullaris occupies a position opposite progressively higher vertebral levels and the nerve roots exiting a particular spinal cord level have to travel farther caudal within the thecal sac toward their eventual exit foramina.
- Final position by 2 months age: The conus medullaris occupies its “adult level,” most commonly opposite or cranial to the L1–L2 disk space, by birth or at most within 2 months after birth.
- Conus below mid-L2 abnormal: Any conus medullaris lying caudal to the mid-body of L2 is considered abnormally low (95% confidence limits) and therefore potentially tethered.