Fetus in fetu

Fetus-in-fetu (FIF) is an extremely rare abnormality that occurs secondary to abnormal embryogenesis in a monochorionic diamniotic pregnancy where a non-viable fetus becomes enclosed within a normally developing fetus.

This condition is thought to have a 2:1 male predominance 2.

Most patients present with an abdominal mass in the 1st year of life.

The exact embryogenesis of fetus-in-fetu is controversial - there are two schools of thought:

  • some propose it occurs from the anomalous embryogenesis in a diamniotic monochorionic twin pregnancy in which a malformed monozygotic twin lies within the body of its fellow twin
  • others consider it to represent a highly organized teratoma

Most occur in the abdomen / retroperitoneal cavity.

Other reported locations include.

  • cranial cavity 4
  • mediastinum 8
  • scrotum

May typically show a calcified abdominal mass with some components favoring fetal parts within the abdomen (or rarely other parts) of another neonate 2,7.

The treatment of choice is total excision. It is considered relatively benign. 

Thought to be originally described by Meckel in the late 18th century 2.

This entity is distinguished from an intrabdominal teratoma by its embryological origin, its unusual location in the retroperitoneal space, its invariable benignity, and by the presence of vertebral organization with limb buds and well-developed organ systems.

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Article information

rID: 14530
System: Obstetrics
Tag: cases, cases
Synonyms or Alternate Spellings:
  • Fetus-in-fetu (FIF)
  • Fetus in fetus

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Cases and figures

  • Case 1: gross pathology
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  • Case 1: axial CT scan
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  • Case 2
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  • Case 3: 3D bone window
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  • Case 4
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