Sirenomelia (mermaid syndrome): an extremely rare congenital malformation (birth defect) that results in partial or complete fusion of the legs. The condition occurs due to abnormalities in the development of the spine and legs. Other organ systems can also be impacted; those affected may also have problems with the digestive systems, kidneys and urinary system. Sirenomelia typically is associated with severe, life-threatening defects and complications, including heart and lung defects. It is often fatal in early infancy. The cause of sirenomelia is unknown, and it affects males more often than females. It is estimated to occur in 1 out of 60,000 to 100,000 births.