A fetus in fetu is alive in the sense that any healthy organ is alive. Its cells are alive, and its organs have a working blood supply from the host. However, a fetus in fetus is not capable of life outside its host: as a rule, it has no (or no functional) brain, heart, lungs, gastrointestinal tract, or urinary tract. A fetus in fetu looks vaguely like a fetus, but is far from being one. That is why some experts think fetus in fetu is not a parasitic twin but rather a variant of fetiform teratoma.
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 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.
Sometimes, however, the host twin survives and is delivered. The parasitic twin grows so large that it starts to harm the host, at which point doctors usually intervene. The condition causes the host to look pregnant, and can occur in both males and females.
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 stomach. 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.
In June 1999, Sanju Bhagat, a man from Nagpur, India, was rushed to a hospital due to difficulty breathing. There, a surgical team removed from his bulging belly a teratoma, a kind of tumor. The report described that the surgical team found a living half-formed "creature" inside Mr. Bhagat's belly.
In November 2006, a Chilean boy in Santiago was diagnosed with fetus in fetu shortly before birth. 
In August 2007, a two month old baby in Baguio from the Philippines named Eljie Millapes was diagnosed with fetus in fetu. The parents of Eljie Millapes were alarmed by the abnormal growth of the stomach of their two-month-old baby. Doctors later discovered that she was suffering from fetus in fetu.
Appearances in popular culture
This article or section contains too many minor or trivial fictional references.
Mere trivia, or references unimportant to the overall plot of a work of fiction, should be deleted. See also what Wikipedia is.
In Billy Cowie’s novel Passenger a man learns how to communicate with his internal parasitic twin sister.
In Stephen King's novel The Dark Half, a writer's past fetus in fetu teratoma, which had been found and removed in childhood, is linked to the later appearance of a murderous "evil twin" version of himself that takes on the pseudonymous identity the writer had used for a dark series of novels, and that he had just retired.
In the X-Files episode "Humbug" the main character has a parasitic twin who detaches from his "host" and kills people around the town.
In season 2, episode 7 of the ABC television program Grey's Anatomy, a man believes he is pregnant; he turns out to have a teratoma.
In the film My Big Fat Greek Wedding, Aunt Voula tells a story about "a lump on the back of [her] neck" that contained "teeth, and a spinal column", probably a teratoma.
The fate and identity of a fetus in fetu is a prominent plot line in the New Zealand drama series The Insider's Guide To Love
In the film Breakin' All the Rules, Nikki describes a teratoma to Quincy.
In The Troublesome Offspring of Cardinal Guzman (1992) by Louis de Bernières, Guzman's demons are caused by a teratoma.
Pinoko, a character in the anime Black Jack, is actually a fetus in fetu extracted by Black Jack from a patient. She was given a plastic exoskeleton, and came to live with Black Jack as his assistant after being rejected by her twin sister. Her name is an obvious reference to Pinocchio, and she even sings a song about him in one episode.
In Patrick O'Brian's multi-novel Aubrey-Maturin series, Stephen Maturin carries a preserved teratoma he had removed from a patient along with him as a prized possession.
In the animated television series Bromwell High, during one of Iqbal's speeches, he appears to be describing the lump removed from himself as having hair in teeth, and revealing it was his twin brother who had been buried inside him all those years.
On Will and Grace, Grace once expressed utter horror at a teratoma with little hair and teeth. She even said "When they poked it, it said 'Ow'."
On the animated television series The Venture Bros., main character Dr. Thaddeus Venture engulfs his brother, Jonas Venture Jr., in the womb. Jonas Jr. later escapes from his brother and attempts to kill him.
Brothers of the Head 2006, is a movie about conjoined twins, one of whom has a third twin, a fetus in fetu, inside his head.
^ Chua, JHY; Chui CH, Sai Prasad TR et al. (2005). "Fetus-in-fetu in the pelvis". Annals of the Academy of Medicine Singapore34: 646-649.
^ Grant P, Pearn JH Foetus-in-foetu. Med J Aust. 1969; 1:1016-1020 — source not consulted; cited here following Hoeffel CC, Nguyen KQ, Phan HT, Truong NH, Nguyen TS, Tran TT, Fornes P. Fetus in fetu: a case report and literature review. Pediatrics. 2000 Jun;105(6):1335-44.
PubMed free full text
^ Basu, A; Jagdish S, Iyengar KR, Basu D (Oct 2006). "Fetus in fetu or differentiated teratomas?". Indian Journal of Pathology and Microbiology49 (4): 563-565.
^ "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
^ "Man With Twin Living Inside Him—A Medical Mystery Classic", ABC News
^ "Chilean boy born with fetus in his stomach", MSNBC, November 24, 2006, accessed June 4, 2007
^ "A Filipino boy born with fetus in his stomach", GMA News, August 3, 2007, accessed November 7, 2007
^ Cowie B (2007) Passenger. Brighton ISBN 9780955400407 published by Idiolect Books