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A rare, lethal, congenital anomaly that may represent the most severe form of caudal dysgenesia and characterized by fusion of the lower limbs (mermaid-like) always associated with severe genitourinary and gastrointestinal anomalies. Furthermore, there is wide phenotipical variability in the musculoskeletal, central nervous system, cardiopulmonary, anomalies present. Pelvic, sacral and spinal defects , internal and external genitalia defects, renal agenesis, absent bladder, rectal/anal atresia are commonly described. Most cases are stillborn or die during, or shortly after, birth. Sirenomelia can be classified on the basis of limb malformations phenotypes. Due to the similarity, the distinction between sirenomelia and caudal regression syndrome, familial caudal dysgenesis and VACTERL is debated.