Fetus in fetu is an extremely rare malformation reported about 100 times in the literature. Fetus in fetu has been defined as the presence of one of the twins, a monozygotic twin, in the body, usually the abdomen, of the other twin.

A fetus in fetu is most frequently located in the retroperitoneal area, but can also be located in other locations as well.  Typically, fetus in fetu presents in early infancy and early childhood, but it can also be diagnosed prior to birth with ultrasonography or other means. 

Depending on where the fetus in fetu is located, surgery is usually required for treatment.