Sirenomelia

Sirenomelia, also known as Mermaid Syndrome, is an extremely rare (1 in about 100,000 live births) disorder in which the legs are fused together, giving them the appearance of a mermaid's tail.

Sirenomelia is usually associated with serious kidney and urogenital abnormalities leading to early death. More than half the cases are still births. Maternal diabetes has been associated with serinomelia and some consider it a severe form of caudal regression syndrome.