Fetus in Fetu

Corinne DeRuiter

Fetus in Fetu

Fetus in fetu is a rare variety of parasitic twins, where the developmentally abnormal parasitic twin is completely encapsulated within the torso of the otherwise normally developed host twin. In the late eighteenth century, German anatomist Johann Friedrich Meckel was the first to described fetus in fetu, which translates to fetus within fetus. Fetus in fetu is thought to result from the unequal division of the totipotent inner cell mass, the mass of cells that is the ancestral precursor to all cells in the body. The unequal division is thought to occur during the formation of the blastocyst, which can also result in parasitic and conjoined twins. Fetus in fetu represents a developmental anomaly that has prompted developmental biologists to further examine the mechanisms for how twins arise.

In the past, scientists speculated that fetus in fetu was the result of a highly developed teratoma rather than the product of abnormal embryonic development. Teratomas are tumors composed of tissues foreign to the area of the body in which they are found, and can include hair, teeth, bone, and organ tissues. Teratomas are the most common type of brain tumor found in infants, and are potentially malignant.

In order to classify an abdominal growth as fetus in fetu rather than a teratoma, several characteristics must be present. Fetus in fetu is a benign growth of embryological origin, and is located behind the abdomen lining of the host twin. To diagnose fetus in fetu there must be evidence of body plan organization, including vertebrae, limb buds, and organ tissues. Abdominal radiographs, computed tomography scans (CT scans), and ultrasonagraphy are helpful technologies for doctors to determine whether a mass is a teratoma or fetus in fetu.

Individuals diagnosed with fetus in fetu generally have the parasitic twin excised, as the mass poses risks to the host twin because the mass may continually increase in size. Increased mass can lead to hemorrhaging in the host twin, and it may increase pressure on the host twin's diaphragm, which can lead to trouble with breathing and potentially suffocation.

With less than 100 known cases worldwide, when fetus in fetu occurs, the event often receives international media attention. With advanced technology, doctors can easily detect fetus in fetu, and parents of infants with this medical anomaly are often encouraged to surgically remove the abnormal mass of tissue to improve the quality of life of the host twin.


  1. Arlikar, Jamir. “Fetus in Fetu: Two Case Reports and Review of Literature,” Pediatric Surgery International 25 (2009): 289–92.
  2. Gangopadhyay, Ajay, Arvind Srivastava, Punit Srivastava, Dinesh Gupta, Shiv Sharma, and Vijayendra Kumar. “Twin Fetus in Fetu in a Child: A Case Report and Review of the Literature,” Journal of Medical Case Reports 4 (2010): 1–7.
  3. Grosfeld, Jay, Donald Strepita, Walter Nance, and Catherina Palmer. “Fetus-in-Fetu: An Unusual Cause for Abdominal Mass in Infancy,” Annals of Surgery 180 (1974): 80–4.
  4. Kim, Jeong-Ah, Jeong Yeon Cho, Young Ho Lee, Mi Jin Song, Jee-Yeon Min, Hak Jong Lee, Byoung Hee Han, Kyung-Sang Lee, Byung Jae Cho, and Yi-Kyeong Chun. “Complications Arising in Twin Pregnancy: Findings of Prenatal Ultrasonography,” Korean Journal of Radiology 4 (2003): 54– 60.
  5. “Man With Twin Living Inside Him—A Medical Mystery Classic.” ABC News: August 23, 2006.
  6. Spencer, Rowena. “Parasitic Conjoined Twins: External, Internal (fetuses in fetu and teratomas), and Detached (acardiacs),” Clinical Anatomy 14 (2001): 428–44.


How to cite

DeRuiter, Corinne, "Fetus in Fetu". Embryo Project Encyclopedia ( ). ISSN: 1940-5030 https://hdl.handle.net/10776/3706


Arizona State University. School of Life Sciences. Center for Biology and Society. Embryo Project Encyclopedia.

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Monday, September 4, 2023 - 17:49

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