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One might think that this is a fairly common phenomenon, as the international press often reports this kind of strange case. Usually, it is a baby, or more rarely an adolescent or young adult, in whom there is a fetus, most often in the abdomen. A mass which may consist of a real skeleton is found during a CT scan or MRI, then extracted by surgery. The fetus thus removed is sometimes even endowed with teeth, hair or limbs, evidence of advanced development. But it also happens that the fetus removed is much smaller and only measures a few centimeters.
We speak here of “parasitic twin”, or more formally of “Fœtus in fetu”.
Note that in November 2010, the ‘Hong Kong Medical Journal‘had reported an extremely rare case where parasitic twins aged about ten weeks were found in a Chinese girl, between the spleen and the kidney. It was therefore initially triplets.
Sometimes, when two twin fetuses intertwine and one takes over the other, this gives rise to strange malformations (arms protruding from the abdomen, two spinal columns, etc.) which are called ischiopagus. We also talk about “cannibal twin”. A phenomenon which, although different, may be reminiscent of that of the Siamese.
Fetus in fetus: what is it?
Fetus in fetu (FIF) is an anomaly in embryonic development in which a fetus is included inside the body of its twin, hence the term “parasitic twin”, Since one“ parasitizes ”the other.
The main theory to explain this phenomenon is that, during the early stages of development, one fetus “absorbs” the other. If the blood system allows it, the “parasitic” fetus can then continue to develop inside its “host” twin.
Note that this phenomenon can only occur during a monochorial bi-amniotic twin pregnancy, that is to say with a single placenta but two amniotic sacs. These are therefore monozygotic twins, or identical twins, from the same egg.
But it sometimes happens that this phenomenon of fetus in fetus is confused with a form of tumorCalled teratoma, where certain cells differentiate to the point of forming different tissues and organ forms (bones, hair, teeth, etc.).
Since it is not always obvious to differentiate a Fetus in fetus from a teratome, doctors sometimes evoke the first phenomenon when in fact it is the second, or vice versa. The presence of an outline of a skeleton, of spine or limbs orients towards the hypothesis of a Fetus in fetus.
The Fetus in fetus would concern one in 500. In other words, out of 500 births, a baby would be the carrier of a parasitic twin. But since it doesn’t always lead to symptoms, it may never be diagnosed, or seen by chance during a CT scan or MRI. To date, less than 000 cases have been described in the medical literature.
Sometimes, however, a fetus in fetus causes symptoms during early childhood, such as abdominal mass and distension, eating difficulties, vomiting or, more rarely, jaundice or respiratory distress.
The “parasitic twin”, not to be confused with the chimera
Note that the phenomenon of Fetus in fetus, or parasitic twin, should not be confused with that of chimera. The latter term refers to a person living with two different DNAs, due to the fusion of two eggs at the embryonic level. A single embryo continues its development, while keeping two different genetic heritages, depending on whether the cells came from one egg or the other. A so-called “chimera” person thus risks having certain parts of the body with a certain genotype, and another characterized by another genotype, but no mass suggesting a partially developed fetus.
Note that this phenomenon can cause many problems during genetic tests, such as a paternity or maternity test.
What is a lithopedion?
Finally, another phenomenon called “lithopédion”May be reminiscent of the fetus in fetus. It is then an ectopic pregnancy, usually abdominal, which has never been diagnosed and which spontaneously stopped without the fetus being expelled. The fetus then calcifies in its mother’s body, then “fossilizes”. In 2013, a mummified fetus was observed in an 84-year-old Colombian woman.
sources:
- https://www.lemonde.fr/blog/realitesbiomedicales/2015/02/11/une-fillette-chinoise-nait-avec-deux-foetus-jumeaux-dans-le-ventre/
- https://www.hkmj.org/abstracts/v21n1/80.htm