Fetus in fetu (or fœtus in fœtu) is a developmental abnormality in which a mass of tissue forms inside the body that resembles a fetus. 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.Chua, JHY; Chui CH, Sai Prasad TR et al. (2005). "Fetus-in-fetu in the pelvis" (PDF). Annals of the Academy of Medicine Singapore 34: 646–9 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.Grant P, Pearn JH (May 1969). "Foetus-in-foetu". Med. J. Aust. 1 (20): 1016–9. PMID 5815070. — source not consulted; cited here following Hoeffel CC, Nguyen KQ, Phan HT, et al. (June 2000). "Fetus in fetu: a case report and literature review". Pediatrics 105 (6): 1335–44. doi:10.1542/peds.105.6.1335. PMID 10835078

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 differentiated teratoma; or, at worst, a high-grade metastatic teratocarcinoma. 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.[1]

There are two main theories about the development of fetus in fetu; one simple, the other complex.

Teratoma theory

Fetus in fetu may be a very highly differentiated form of dermoid cyst, itself a highly differentiated form of mature teratoma.Basu A, Jagdish S, Iyengar KR, Basu D (October 2006). "Fetus in fetu or differentiated teratomas?". Indian J Pathol Microbiol 49 (4): 563–5. PMID 17183856.

Parasitic twin theory

Fetus in fetu may be a parasitic twin fetus growing within its host twin. 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 its survival depends on the survival of its 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, it usually dies before birth.

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.

• Alamjan Nematilaev was the surviving host of a fetus in fetu. In 2003, aged 7, his school physician in Kazakhstan referred him to 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 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."How did that boy end up with his twin growing inside him?" The Guardian, July 17, 2003, accessed June 4, 2007"The Boy Who Gave Birth to His Twin", Channel 4, December 2003, accessed June 4, 2007

• 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."Man With Twin Living Inside Him—A Medical Mystery Classic", ABC News
• In March 2006, Doctors in Pakistan removed two fetuses from inside a two-month-old baby girl.MSNBC,March 2006

• In November 2006, a Chilean boy in Santiago was diagnosed with fetus in fetu shortly before birth."Chilean boy born with fetus in his stomach", MSNBC, November 24, 2006, accessed June 4, 2007

• 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."A Filipino boy born with fetus in his stomach", GMA News, August 3, 2007, accessed November 7, 2007

• In January 2008, a two-month-old baby in Medan, Indonesia named Afiah Syafina was diagnosed with tumor in her abdomen. After the operation was done at January 19, 2008, the results were startling enough. The suspected tumor was a five-month-old 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 Athens 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.Your Own Twin", ABC News

• In October, 2008, a doctor in the USA removed a brain tumor 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.'I've never seen anything like it,' doctor says of newborn's brain surgery, The Gazette (Colorado Springs), 2008-12-14, retrieved 2008-12-18"Baby born with 'foot in brain'". BBC (London). December 19, 2008

• 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 have been told she can return home at the end of the month having made a full recovery.'Pregnant' one-year-old girl has unborn twin's foetus removed

• Many of Philip K. Dick's works have an "unborn" twin, such as Dr. Bloodmoney.
• Jonas Venture Jr. of The Venture Bros. cartoon series is a fetus in fetu who manages to survive extraction from his encapsulating twin, Doctor Thaddeus Venture.
• Stephen King's The Dark Half proposes the possible delayed awakening of an unborn twin in the protagonist's body and mind.
• Black Jack by Osamu Tezuka has a chapter dealing with the condition.
• 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."Times Fiction Reviews",
• 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.