Anteroposterior abdominal radiograph shows a soft-tissue mass in the right hemiabdomen. The mass contains calcified osseous-appearing structures of varying sizes and shapes.
The postoperative specimen shows a fairly well developed fetus lying on its back, with rudimentary digits.
Computed Tomography scan of the patient's abdomen reveals a large retroperitoneal soft-tissue mass. There are long hyperdense opacities that resemble fetal bones.
Fetus in fetu (or foetus in foetu) is a developmental abnormality in which a mass of tissue resembling a fetus forms inside the body. There are two theories of origin concerning "fetus in fetu". One theory is that the mass begins as a normal fetus but becomes enveloped inside its twin. The other theory is that the mass is a highly developed teratoma. "Fetus in fetu" is estimated to occur in 1 in 500,000 live births.
A fetus in fetu can be considered alive, but only in the sense that its component tissues have not yet died or been eliminated. Thus, the life of a fetus in fetu is akin to that of a tumor in that its cells remain viable by way of normal metabolic activity. However, without the gestational conditions in utero with the amnion and placenta, a fetus in fetu can develop into, at best, an especially well differentiatedteratoma; or, at worst, a high-grade metastaticteratocarcinoma. In terms of physical maturation, its organs have a working blood supply from the host, but all cases of fetus in fetu present critical defects, such as no functional brain, heart, lungs, gastrointestinal tract, or urinary tract. Accordingly, while a fetus in fetu can share select morphological features with a normal fetus, it has no prospect of any life outside of the host twin. Moreover, it poses clear threats to the life of the host twin on whom its own life depends.
Theories of development
There are two main theories about the development of fetus in fetu; one simple, the other complex.
Fetus in fetu may be a parasitic twinfetus growing within his or her hosttwin. Very early in a monozygotic twin pregnancy, in which both fetuses share a common placenta, one fetus wraps around and envelops the other. The enveloped twin becomes a parasite, in that his or her survival depends on the survival of his or her host twin, by drawing on the host twin's blood supply. The parasitic twin is anencephalic (without a brain) and lacks some internal organs, and as such is almost always unable to survive on its own. As the normal twin has to "feed" the enveloped twin from the nutrients received over a single umbilical cord, he or she usually dies before birth.
Examples from history and in the media
An early example of the phenomenon was described in 1808 by George William Young. 
Cases of fetus in fetu sometimes attract worldwide media attention. These cases are a small minority of the known cases and rarely overlap with cases reported in the medical literature, but they are widely accessible.
In June 1999, the case of Sanju Bhagat a man from Nagpur, India attracted attention for the length of time (36 years) he had carried his parasitic "twin" inside his body, and the size of the growth. As Bhagat had no placenta, the growth had connected directly to his blood supply. Much of the size of the swelling was accumulated sebaceous secretions and hair shed from the parasite's skin.
Alamjan Nematilaev was the surviving host of a fetus in fetu. In 2003, aged 7, his school physician in Kazakhstan referred him to a hospital after movements were detected in the boy's enlarged abdomen. An operation intended to remove a cyst uncovered the fetus of Alamjan's identical twin brother, which had lived as a parasitic growth inside the boy throughout his entire life. The fetus was comparatively highly developed, with hair, arms, fingers, nails, legs, toes, genitals, a head, and a vague approximation of a face.
In March 2006, Doctors in Pakistan removed two fetuses from inside a two-month-old baby girl.
In November 2006, a Chilean boy in Santiago was diagnosed with fetus in fetu shortly before birth.
In August 2007, two-month-old Eljie Millapes from Baguio, the Philippines, was diagnosed with fetus in fetu. The parents of Millapes were alarmed by the abnormal growth of the abdomen of their baby. Doctors later discovered that she was suffering from fetus in fetu.
In January 2008, a two-month-old baby in Medan, Indonesia named Afiah Syafina was diagnosed with a tumor in her abdomen. An operation was done at January 19, 2008 which revealed the suspected tumor to actually be a fetus.
In May 2008, a two-inch (5 cm) embryo was removed from the belly of a 9-year-old girl at Larissa General Hospital in Greece after she was diagnosed with a tumor on the right side of her belly. The embryo was a fetus with a head, hair and eyes, but no brain or umbilical cord.
On June 30, 2008, in China, a newborn baby with this condition had an extra penis growing out of its back. Doctors spent three hours removing it, and the child was fine afterward.
In October 2008, a doctor in the United States removed a brain tumor from Sam Esquibel that consisted of a foot and other body parts. This may have been a case of fetus in fetu or it may have been a teratoma.
In September 2009, one-year-old Kang Mengru from China was "pregnant" with her own twin. After 10 hours of complex surgery to remove the fetus, Kang's family was told she could return home at the end of the month. She made a full recovery. 
In August 2011, in Mansoura, Egypt, Professor Ibrahim Dawoud removed the retained twin of an 18 year old male after a three-hour surgery.
In September 2011, a 41-year-old man in Arjawinangun Public Hospital, Cirebon, Indonesia was diagnosed with a tumor in his abdomen. An operation was performed on October 4, 2011, which revealed the suspected tumor to actually be a fetus.
In the 1990 film Total Recall, George/Kuato, the mutant leader of the resistance attached to his unmutated brother
Black Jack by Osamu Tezuka has a chapter dealing with the condition. After being removed, the fetus in fetu is given a prosthetic body and becomes the title character's sidekick for the rest of the series.
The concept is also the central theme of the novel Passenger by Billy Cowie (published by Old Street Publishing 2008) where the main character Milan discovers his sister Roma is embedded in himself and spends the remainder of the book attempting to communicate with her.
My Big Fat Greek Wedding's Aunt Voula casually mentions a "lump" that turned out to be her "unborn twin" in passing after the Miller family is accepted as in-laws to the lead character.
The TV show X-Files episode "Humbug" (Season 2) centers on a fetus in fetu that is able to leave the host-twin's body for a limited time.
The comedy/horror film Basket Case centers on a man's murderous parasitic brother.
In the comedy series Community (TV series), Chang states that he consumed his twin sibling's fetus un utero.
In the first episode of the Korean drama Dr. Jin, a patient undergoing surgery to remove a brain tumor surprised everyone when instead of a tumor, something that appeared to be a fetus in fetu or perhaps a teratoma was removed from his brain.
In the animated comedy South Park, Nurse Gollum is said to have a fictional but related condition called "conjoined twin myslexia".
^Chua, JHY; Chui CH, Sai Prasad TR et al. (2005). "Fetus-in-fetu in the pelvis" (PDF). Annals of the Academy of Medicine Singapore34: 646–9.Cite uses deprecated parameters (help)