title: Fetus-in-fetu in the pelvis: report of a case and literature review.
abstract: BACKGROUND
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Padediatric Surgery, KK Women's and Children's Hospital, Singapore. joyce.chua.h.y@singhealth.com.sg
authors.forename: J H Y
authors.initials: JH
authors.lastname: Chua
authors.forename: C H
authors.initials: CH
authors.lastname: Chui
authors.forename: T R
authors.initials: TR
authors.lastname: Sai Prasad
authors.forename: A S
authors.initials: AS
authors.lastname: Jabcobsen
authors.forename: A
authors.initials: A
authors.lastname: Meenakshi
authors.forename: W S
authors.initials: WS
authors.lastname: Hwang
mesh_terms: D003456:Cryptorchidism / Q000175:diagnosis; D003937:Diagnosis, Differential; D005260:Female; D005315:Fetal Diseases / Q000175:diagnosis / Q000601:surgery; D005333:Fetus / Q000002:abnormalities* / Q000601:surgery; D006801:Humans; D007231:Infant, Newborn; D010535:Laparoscopy; D008297:Male; D011247:Pregnancy; D011859:Radiography; D012187:Retroperitoneal Space / Q000000981:diagnostic imaging; D013724:Teratoma / Q000175:diagnosis; D016216:Ultrasonography, Prenatal