01/19/2026
PMID: 33628563 Fetus in fetu (FIF) is an extremely rare developmental condition. It occurs in about 1 in 500 000 live births, and around 200 cases have been reported in medical literature. FIF is usually retroperitoneal, but it has been reported at various sites. Majority of cases have been described in neonates and children, and only few cases have been reported after the age of 15 years and most of them are male.Fetus in fetu occurs secondary to abnormal embryogenesis in a monochorionic diamniotic pregnancy in which a malformed parasitic twin is found inside the body of its partner as an abdominal fetiform mass. However, other sites like the mediastinum, neck, sacrococcygeal region, back, scrotal sac, cranial cavity, and oral cavity have also been reported to contain FIF. Most commonly, it presents with a single parasitic fetus as in our case; however, multiple fetuses ranging from 2 to 5 have also been reported. Symptoms relate to the mass effect. Organs of different systems can be found in these fetuses. Commonly noticed organs are the vertebral column and limbs. However, other organs such as the ribs, central nervous systems, gastrointestinal tract, vessels, and occasionally thymic tissues can also be seen. Other uncommon organs reported are thyroid, parathyroid, pancreas, spleen, kidney, adrenal, te**is, ovaries, urinary bladder, tongue, salivary glands, lymph nodes, trachea, and teeth.Fetus in fetu and well-formed teratoma having all three germ layer components are a matter of dispute for their independent existence. “Willis criteria” explain the differences between the two, on the basis of an axial skeleton with vertebral axis development (explaining embryological development passing through the stage of primitive streak) and an appropriate arrangement of other organs and limbs with respect to the axis in FIF. To be called fetus in fetu, the mass must demonstrate true organogenesis.