Mermaid Syndrome, also known as Sirenomelia, is characterized by the legs being fused together to create a mermaid-like effect. The condition occurs in about one in 100,000 pregnancies. In many cases, Mermaid Syndrome is fatal because kidneys and urinary bladder are not properly developed in utero. There is a less severe form of Mermaid Syndrome with only a handful of children surviving after birth. In some cases, parents choose to have the legs separated while the children are young. In others, the children continue to live with fused legs.

Children who live beyond birth with Mermaid Syndrome typically have no defects other than bone fusion in the lower part of the body. At least one case is known where a child did have severe internal defects and lived. Shiloh Pepin was born with no bladder, no uterus, six inches of large colon, no vagina  and only one-quarter of a kidney. The child lived 10 years before passing away from pneumonia.