Fetus in fetu

Last revised by Dr Mostafa El-Feky on 25 Oct 2021

Fetus-in-fetu 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.

Fetus-in-fetu is very rare, with an incidence of 1/500,000 live births 11. This condition is thought to have a 2:1 male predominance 2.

Most patients present with an abdominal mass in the first year of life 11. There also may be symptoms of mass effect, such as difficulty feeding 11.

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

In the vast majority of cases, there is only a single fetoid form present, however rarely there may be multiple, with up to 11 fetoid forms having been reported in one case 12.

Most occur in the abdomen, with 80% in the retroperitoneal cavity 10,11. Other rarer reported locations include:

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. This is best seen on cross-sectional imaging rather than plain radiograph 11.

Different fetal parts may be appreciated 11:

  • vertebral column (most common - ~90%)
  • limbs (~80%)
  • central nervous system (~55%)
  • gastrointestinal tract (~45%)
  • vasculature (~40%)
  • genitourinary tract (~25%)

The treatment of choice is total surgical excision 11. It is considered relatively benign 11

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 11.

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Cases and figures

  • Case 1: gross pathology
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  • Figure 2: 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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  • Case 5: specimen x ray
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  • Case 6
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