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A baby growing inside of another? This is a medical mystery, one of its kind.

What is Fetus in Fetu?

“Fetus in Fetu” is a rare pregnancy complication in which a fetus (or a mass of tissue tightly resembling a fetus) is present inside of another fetus. Fetus in fetu can be characterized as being alive or dead: it is considered “alive” when the tissues of the inside fetus have a viable blood supply, and it is “dead” when the tissues are non-functioning. In viable fetus in fetu and depending on the level of development of the internal organs, the internal fetus can present with a fully functioning gastrointestinal tract, urinary tract or even respiratory tract. However, its survival is strictly dependent on the blood supply of its host twin.

Theories of Development

There are two main theories that have been postulated in attempt to explain the occurrence of this peculiar medical condition.

The Teratoma Theory: First, fetus in fetu could result from a highly differentiated teratoma: by definition, a teratoma is a germ cells tumor that features cells from the ectoderm, mesoderm and endoderm. Those three layers are the cornerstone, the rock from which all the organs and tissues are derived during fetal development. This would explain why on biopsy, a teratoma can display features such as hair, teeth, bone and blood vessels. Teratomas are benign tumors by default, but also have the possibility of being malignant.

The Parasitic Twin Theory: this is the second hypothesis that explains Fetus in Fetu. According to the Parasitic Twin Theory, during the early fetal development of monozygotic twins (those that share a placenta), one twin benefits from more blood supply than the other, and therefore grows faster and much better than the other. Eventually, due to the smaller size of the unfortunate twin, the dominant twin surrounds him and engulfs him. 

The survival of the engulfed and enveloped twin strongly depends on the blood supply of the bigger twin, and this makes the engulfed twin become the “parasite”.

Fetus in fetu has made a lot of buzz in the social media, due to the rare yet fascinating character of this medical mystery. However, it is important to emphasize that the condition can be very dangerous for the twin who is carrying his parasitic twin inside of him; as it can cause compression of internal organs, internal bleeding and in the event that it is a malignant teratoma, it can result in accelerated cellular division and cancer metastasis.

The Difference between a Fetus in Fetu and a Teratoma

Some key differences between fetus in fetu and teratoma have been highlighted by several researchers to demonstrate that the condition (fetus in fetu) is indeed a truly isolated pregnancy complication that is not to be associated with teratomas.

For starters, unlike teratomas, fetus in fetu do not have any malignancy potential.

That is, the risk of a fetus in fetu developing into cancer is extremely low, if not inexistent. In addition, fetus in fetu develop in the upper body, whereas teratomas tend to develop in the sacral and coccygeal regions. (Nevertheless, there is a possibility of fetus in fetu developing in the testes and the scrotum as well). Lastly, the presence of a vertebral column confirms the diagnosis of fetus in fetu, as teratomas do not present with vertebral column.

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