• J H Y Chua, C H Chui, T R Sai Prasad, A S Jabcobsen, A Meenakshi, and W S Hwang.
• Department of Padediatric Surgery, KK Women's and Children's Hospital, Singapore. joyce.chua.h.y@singhealth.com.sg
• Ann Acad Med Singap. 2005 Nov 1; 34 (10): 646-9.

IntroductionFetus-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.Clinical PictureWe 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.OutcomeThe 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.ConclusionsAlthough 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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