Caudal Regression Syndrome (CRS) is a rare fetal complication of diabetic pregnancy, which can result in long-term neurological, urologic, and orthopedic complications. Although the exact teratogenic mechanism is not known, hyperglycemia appears to play a crucial role as a teratogen, and therefore, stringent control of diabetes preconceptually and in early pregnancy is presumed to reduce the risk of occurrence. We report an unusual case of CRS affecting only one of a set of monozygotic twins, suggesting that as yet, unidentified factors other than hyperglycemia are included in its causation.