Sirenomelia is a birth defect in which affected infants are born with a single lower extremity or with two legs that are fused together. The symptoms and physical findings associated with the condition vary greatly among affected individuals and may include malformations of the spine and skeletal system (commonly with vertebrae either absent or defective); absent or underdeveloped internal and external sex organs, rectum, kidneys and/or bladder; closed rectal opening (imperforate anus); and other abnormalities of the lower gastrointestinal tract. The exact cause is unknown, but it is believed to result from irregularities in early development of the blood circulating system within the embryo. Surgery has been successful in separating joined legs.