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.