Sacral agenesis, lumbosacral agenesis, and caudal regression syndrome are three terms that are often used interchangeably. Sacral agenesis refers to a condition in which the sacrum (the lowest portion of the spine that forms the joint with the hips) fails to form (agenesis) during fetal development. Lumbosacral agenesis describes a more extensive form of this condition in which one or more of the lumbar vertebrae may also be missing. Caudal regression syndrome refers to the same condition — the missing portions of the spine (the caudal regression) — and the syndrome of conditions that often accompany it.