Distal end of the spinal cord

The conus medullaris (aka conus terminalis) is the tapered, lower end of the spinal cord.
Ends at L1 (T12-L3) in adults
Ends at L3 in neonates
During fetal development, cord extends to end of the sacrum
After the spinal cord tapers out, the spinal nerves continue to branch out diagonally, forming the cauda equina.
Thecal sac or dural sac is the membranous sheath (theca) or tube of dura mater that surrounds the spinal cord and the cauda equina, contains the cerebrospinal fluid.
Ends at S2-3 in adults
Ends at S3-4 in neonates
During development, the vertebral column grows faster than the spinal cord.