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Ann Acad Med Singapore. 2005 Nov;34(10):646-9.

Fetus-in-fetu in the pelvis: report of a case and literature review.

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Department of Padediatric Surgery, KK Women's and Children's Hospital, Singapore.



Fetus-in-fetu is an extremely rare condition in which a malformed fetus is found in the body of its twin. To our knowledge, fewer than 100 cases have been reported. Wide variations of presentation have been described, although its embryo-pathogenesis and differentiation from a teratoma have not been well established.


We describe a male neonate with a fetoid-like mass in his pelvis associated with bilateral undescended testes. The mass was detected on prenatal ultrasound scans. The diagnosis of fetus-in-fetu was considered prenatally and confirmed on a computed tomography scan after birth.


The mass was successfully excised. Histological examination, accompanied by a review of the literature, confirmed that the mass had features consistent with a fetus-in-fetu.


Although an extremely rare clinical entity, fetus-in-fetu can be diagnosed prior to surgery with current imaging modalities. When it arises in the retroperitoneum of a male infant, it can hinder the descent of the testes. Complete excision is curative.

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