Fetus in fetu
fetus-like tissue mass within a fetus, abnormality of twin pregnancy
Fetus in fetu (or foetus in foetu) is a rare developmental abnormality in which a mass of tissue resembling a fetus forms inside the body of a true fetus. An early example of the phenomenon was described in 1808 by George William Young.
There are two hypotheses for the origin of a fetus in fetu. One hypothesis is that the mass begins as a normal fetus but becomes enveloped inside its twin. The other hypothesis is that the mass is a highly developed teratoma. Fetus in fetu is estimated to occur in 1 in 500,000 live births (0.0002%).
Classification as life
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.
Begin with the source’s own compact description: “Fetus in fetu” is fetus-like tissue mass within a fetus, abnormality of twin pregnancy. The dossier treats that line as a proposition to test through Fetus, fetu and fetus-like, not as a finished interpretation.
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The citation trail is more important than the brevity of the summary: it shows where individual claims can be examined in context. The source revision retrieved here is dated May 14, 2026. The linked authority identifier is Q1435125. None of the 0 selected statements returned an explicit reference. The first chronological checks are 1808.
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This entry incorporates text from “Fetus in fetu” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.