Health What to know about mermaid syndrome Medically reviewed by  Karen Gill, M.D.  —  Written by Lowri Daniels   What it is Causes Signs and symptoms Treatment Risk factors Outlook Summary Mermaid syndrome, or sirenomelia, is an extremely rare but very serious condition that is present from birth (congenital). It is called mermaid syndrome because the baby’s legs are partially or completely fused. Babies born with mermaid syndrome often also have serious internal problems that affect multiple organs in the body. Keep reading to learn more about mermaid syndrome, including the risk factors, causes, and treatment options. What is mermaid syndrome? Share on Pinterest Image credit: Cavan Images / Getty Images. Mermaid syndrome is a serious condition in which a baby’s legs are completely or partially fused from birth. It is often fatal early in life. Mermaid syndrome is, however,  extremely rare . In fact, it is so rare that the exact incidence is hard to measure. Experts believe that it o