Caudal regression syndrome, or sacral agenesis (or hypoplasia of the sacrum), is a rare birth defect. It is a congenital disorder in which the fetal development of the lower spine—the caudal partition of the spine—is abnormal. It occurs at a rate of approximately one per 60,000 live births. Some babies are born with very small differences compared to typical development, and others have significant changes. Most grow up to be otherwise normal adults who have difficulty with walking and incontinence.