Sirenomelia, alternatively known as Mermaid Syndrome is a very rare congenital deformity in which the legs are fused together, giving the appearance of a mermaid. This condition is found in approximately one out of every 70,000 live births (about as rare as conjoined twins) and is usually fatal within a day or two of birth because of complications associated with abnormal kidney and bladder development and function. It results from a failure of normal vascular supply from the lower aorta in utero. Maternal diabetes has been associated with caudal regression syndrome and sirenomelia, although this association is not generally accepted. VACTERL-H is an expanded form of the VACTERL association that concludes that this diagnosis is a less severe form of sirenomelia.The disorder was formerly thought to be an extreme case of Caudal regression syndrome; however, it was reclassified to be considered a separate condition.

Syndactyly is a characteristic of Apert syndrome, Poland syndrome, Jarcho-Levin syndrome, oral-facial-digital syndrome, Pfeiffer syndrome, and Edwards syndrome

Stockholm syndrome.

The Koro syndrome is a culture-specific syndrome. The individual with Koro syndrome has an overpowering belief that his or her genitals are retracting and disappearing.