Philippe Jeanty, MD, PhD Luís Flåvio de Andrade Gonçalves, MD Sandra Rejane Silva, MD

[1] Florianopolis, SC Brazil, [2] Sao Paulo — SP [3] Nashville, TN jeanty@pov.net

Adapted and reproduced with permission from TKI Medcon Inc. http:/www.tki.com/


A fetus-in-fetu is an encapsulated, pedunculated vertebrate tumor. It represents a malformed monozygotic, monochorionic diamniotic parasitic twin included in a host (or autosite) twin. Characteristically the fetus-in-fetu complex will be composed of a fibrous membrane (equivalent to the chorioamniotic complex) that contains some fluid (equivalent to the amniotic fluid) and a fetus suspended by a cord or pedicle. The presence of a rudimentary spinal architecture is used to differentiate a fetus-in-fetu from a teratoma, since teratomas are not supposed to develop through the primitive streak stage (12-15 days). This last criterion has been considered too stringent by many authors who regard a rudimentary body architecture (metameric segmentation, craniocaudal and lateral differentiation, body coelom, "gestational sac"), or the presence of an associated fetus-in-fetu as equivalent criteria. Although teratomas can achieve striking degrees of differentiation by the inductive effect of adjacent tissues on one another, they do not present the criteria mentioned above.



A fetus-in-fetu recovered from a 16-year old boy (From Highmore, 1815)

Sonographic features

The few cases detected prenatally all presented as a complex mass. The general appearance is a well-delineated capsule, with an echogenic mass suspended in fluid or partially surrounded by fluid. Occasionally, the diagnosis can be suggested by the recognition of a rudimentary spine.



Longitudinal and axial section of a fetus-in-fetu. Notice the faintly visible spinal organization on the longitudinal view (Courtesy Dr. Patricia Dix).

Differential diagnosis

When discovered in a newborn child during physical examination, the differential diagnosis includes all the common masses such as Wilms" tumor, hydronephrosis, and neuroblastomas. Prenatally, the main differential diagnosis is with teratoma. Teratomas are disorganized congregations of pluripotential cells from all three primitive tissue layers. By differentiation and induction, they can achieve striking organization, with examples of several organs being well formed. However, teratomas do not have vertebral segmentation, craniocaudal and lateral differentiation, body coelom or systemic organogenesis. Thus the presence of a mass with a spinal organization and surrounded by fluid suggests the correct diagnosis.

When spinal structures are not present, most authors have considered that the diagnosis of fetus-in-fetu can still be made when the alternate criteria described under Definition are found. These criteria are sufficiently restrictive that even well-organized teratomas cannot fulfill all of them.

Teratomas have a definite malignant potential, a feature that has not been reported in fetus-in-fetu. Teratomas occur predominantly in the lower abdomen, not the upper retroperitoneum. Yet, the coexistence of a fetus-in-fetu and a teratoma as well as the occurrence of a teratoma 14 years after removal of a twin fetus-in-fetu have been reported, supporting the older hypothesis of a continuum between twin and teratoma. Cases of sacrococcygeal fetus-in-fetu should probably be regarded and treated as teratoma, because of the high incidence of teratoma in this region.

Ectopic testicles have a higher incidence of germ cell tumors, and the differentiation between fetus-in-fetu and teratoma is particularly important.

Although the characteristics of intracranial teratoma differ from those of intracranial fetus in fetu, Wakai found, in a large review of 245 intracranial teratomas, that there are some transitions between certain teratoma and fetus-in-fetu.

In the older literature, several descriptions of fetus-in-fetu were too vague to be acceptable by current criteria. For example, the case reported by Phillips does not unequivocally suggest the criteria described above and therefore should probably be considered a teratoma.

Some have argued that fetus-in-fetu should be considered as teratomas since they do not evolve into lithopedion like fetuses of abdominocyesis. That argument is probably not valid since in abdominocyesis the antigen complements of the host and fetus are different, which contrasts with fetus-in-fetu.

Associated syndromes



  1. Jeanty P, Caldwell Kay and Dix P: Fetus in fetu. The Fetus 6518:1, 1992
  2. Hill LM, Kislak S, Ahdab-Barmada M: Fetus in fetu. The Fetus 6518:11, 1993
  3. Nicolini U, Dell"Agnola CA, Ferrazzi E et al: Ultrasonic prenatal diagnosis of fetus in fetu. JCU 11:321-2, 1983
  4. Bomsel F, Stroh-Marcy A, Thabaut S, et al: Fetus in fetu: un cas dépisté par échographie anténatale. JEMU 6:163-8,1985
  5. Heifetz SA, Alrabeeah A, Brown BS et al: Fetus in fetu: a fetiform teratoma. Pediatr Pathol 8: 215-26, 1988.
  6. Chitrit Y, Zorn B, Scart G et al: Foetus in foetu surrenalien: un cas evoque par l"echographie prenatale. Revue de la litterature. J Gynecol Obstet Biol Reprod (Paris) 19: 1019-22, 1990.
  7. Martinez Urrutia MJ, Lopez Pereira P et al: Abdominal mass: "fetus in fetu". Acta PÊdiatr Scand 79: 121-2, 1990.
  8. Sada I, Shiratori H, Nakamura Y: Antenatal diagnosis of fetus in fetu. Asia Oceania J
  9. Obstet Gynaecol 12: 353-6, 1986.
  10. Grant P, Pearn JH: Fetus in fetu. Med J Aust 1: 1016-9, 1969.
  11. Kafka J: Teratoma: an explanation of its cause based on the organizer theory of embryology. Arch Pathol 21:756, 1936.
  12. Carles D, Alberti EM, Serville F, et al: Fetus-in-fetu et monstre acardiaque: un mécanisme morphogénique commun explique-t-il les similitudes de ces deux malformations? Arch Anat Cytol Pathol 39: 77-82, 1991.
  13. Chateil JF, Diard F, Bondonny JM, et al: Foetus in foetu testiculaire intraperitoneal. Pediatrie 45: 255-7, 1990.
  14. Alpers CE, Harrison MR: Fetus-in-fetu associated with an undescended testis. Pediatr Pathol 437-46, 1985.
  15. Corona Reyes D, Navarro Cruz RA, Toxtle Tlamani R, et al: Fetus-in-fetu originado en un testiculo criptorquidico. Bol Med Hosp Infant Mex 39: 680-4, 1982.
  16. Potter EL: Pathology of the fetus and newborn (2nd Ed.) Chicago, Year Book Med Pub 1962: p.183-7.
  17. Willis RA: The structure of teratoma. J Pathol Bacteriol 40:1-36, 1935.
  18. Young GW: Case of a fetus found in the abdomen of a boy. Med Chir Trans 1:234, 1809.
  19. Highmore N: Case of a fetus found in the abdomen of a young man, at Sherborne, in Dorsetshire. Royal Collegue of Surgeons, London, 1815 30pp 2pl.
  20. Phillips E: Account of a case in which parts of a fetus were found in a tumour situated in the abdomen of a girl two and a half year old. Medico-chirugical Transactions, 6:124-7, 1815,
  21. Taylor S: Case of included ovum. Transactions of the Pathological Society of London. 38:440-4, 1887.
  22. McNutt WF: Case Report. Pacific Med J 37:118, 1894.
  23. von Haberer H: Operativ entferater Fötus in Föto. Zentralbl Chir 66:840, 1939.
  24. Eng HL, Chuang JH, Lee TY, Chen WJ: Fetus in fetu: a case report and review of the literature. J Pediatr Surg 24: 296-9, 1989.
  25. Kimmel DL, Moyer EK, Peale AR, et al: A cerebral tumor containing five human fetuses. A case of fetus in fetu. Anat Rec 106:141-65, 1950.
  26. Povysilova V: Encranius s mnohotnymi rudimentarnimi fetus-in-fetu u nedonoseneho chlapce. [Encranius with multiple rudimentary fetus-in-fetu in a premature boy]. Cesk Patol 19: 49-54, 1983.
  27. Brines RJ: A large teratoma containing rudimentary arm bones and a hand. JAMA 103:338, 1934.
  28. Gale CW, Willis RA: A retroperitoneal digit-containing teratoma. J Pathol Bacteriol 56:403-9, 1944.
  29. Wollin E, Ozonoff MB: Serial development of teeth in an ovarian teratoma. N Engl J Med 265:897-8, 1961.
  30. Oberman B: Intracranial teratoma replacing brain: report of a case. Arch Neurol 11:423-6, 1964.
  31. Afshar F, King TT, Berry CL: Intraventricular fetus in fetu. Case report. J Neurosurg 56:845-9, 1982.
  32. Bernal Sprekelsen JC, Bernal Cascales M: Fetus in Fetu: Ein Fallbericht einer extrem seltenen Ursache von Bauchtumoren beim Sling. Z Kinderchir 45: 317-8, 1990.
  33. Bomsel F, Stroh-Marcy A, Thabaut S et al: Fetus in fetu: un cas dépisté par échographie anténatale. JEMU 6:163-8,1985.
  34. Boyce MJ, Lockyer JW, Wood CBS: Fetus in fetu: serological assessment of monozygotic origin by automated analysis. J Clin Pathol 25:793-8, 1972.
  35. Broghammer BJ, Wolf RS, Geppert CH: The included twin or fetus in fetu—a case report. Radiology 80:844-6, 1963.
  36. Burtner CD, Conn AG: Fetus in fetu: case report. W V Med J 84: 123-5, 1988.
  37. Chi JG, Yoon SL, Park YS et al: Fetus in fetu: report of a case. Am J Clin Pathol 82:115-9, 1984.
  38. Du Plessis JPG, Winship WS, Kirstein JDL: Fetus-in-fetu and teratoma: a case report and review. S Afr Med J 48:2119-12, 1974.
  39. Farris JM, Bishop RC: Surgical aspects of included twins. Surg 28:443-8, 1950.
  40. Fujikura T, Hunter WC: Retroperitoneal fetus in fetu. Obstet Gynecol 13:547-54, 1959.
  41. Galatius-Jensen F, Rah DH, Uhm IK, el al: Fetus in fetu. Br J Radiol 38:305-8, 1965.
  42. George V, Khanna M, Dutta T: Fetus in fetu. J Pediatr Surg. 18: 288-9, 1983.
  43. Griscom T: The roentgenology of abdominal masses. AJR 93:447-63, 1965.
  44. Grosfeld JL, Stepita DS, Nance WE et al: Fetus in fetu: an unusual cause for abdominal mass in infancy. Ann Surg 180:80-84, 1974.
  45. Gross RE, Clatworthy HW: Twin fetuses in fetu. J Pediatr 38:502-8, 1951.
  46. GĂŒrses N, GĂŒrses N, Bernay F: Twin fetuses in fetu and a review of the literature. Z Kinderchir. 45: 319-22, 1990.
  47. Janovski NA: Fetus in fetu. J Pediatr 61:100-4, 1962.
  48. Kakizoe T, Tahara M: Fetus-in-fetu located in the scrotal sac of a newborn infant: a case report. J Urol 107:506-8, 1972.
  49. Karasimbarao KL, Mitra SK, Pathak IC: Sacrococcygeal fetus in fetu. Indian Pediatr 21:820-2, 1984.
  50. Knox AJS, Webb AJ: The clinical features and treatment of a fetus in fetu: two cases reports and a review of the literature. J Pediatr Surg 7: 434, 1972.
  51. Lal M: A fetus in the abdomen of a young boy. Med J Malaya 25:307-310, 1971.
  52. Lamabadusarya SP, Atukrale AW Soysa PE et al: A case of fetus in fetu. Arch Dis Child 47:305-7, 1972.
  53. Lee EYC: Fetus in fetu. Arch Dis Child 40:689-93, 1965.
  54. Lewis PH: Fetus-in-fetu and the retroperitoneal teratoma. Arch Dis Child 36:220-6, 1961.
  55. Lord JM: Intra-abdominal fetus in fetu. J Pathol Bacteriol 72:627-41, 1956.
  56. Maxwell RW: Endocyme fetus in a Fiji infant. Br Med J 732-3, 1947.
  57. Montupet P, Sinico M, Soulier YA et al: Fetus in fetu: rapport de 2 cas et analyse de la littérature. Chir Pediatr 25:37-42, 1984.
  58. Nadimpalli VR, Reyes H, Manaligold JR: Retroperitoneal teratoma with fetuses. Teratology 39:233-6, 1989
  59. Nocera RM, Davis M, Hayden CK Jr et al: Fetus-in-fetu. AJR 138: 762-4, 1982.
  60. Numanoglu I, Yavuz Gödemir A et al. : Fetus in fetu. J Pediatr Surg 5:472-3, 1970.
  61. Ouimet A, Russo P: Fetus-in-fetu or not? J Pediatr Surg. 24: 926-7, 1989.
  62. Prasad KR, Rai VK, Chowdhary DK: Fetus in fetu. J Indian Med Assoc 77:134-6, 1981.
  63. Rastogi V, Singhal PK, Taneja SB: Fetus in fetu. Indian Pediatr. 25: 584-6, 1988.
  64. Sangvichien S, Sutthiwan P: Fetus-in-fetu and teratoma: their genesis and the formation of vertebral column. J Med Assoc Thai. 65: 505-10, 1982.
  65. Sutthiwan P, Sutthiwan I, Tree-trakan T: Fetus in fetu. J Pediatr Surg. 18: 290-2, 1983.
  66. Wakai S: On the origin of intracranial teratomas. No To Shinkei. 41: 947-53, 1989.
  67. Wiel MA, Tortollo G, Butti et al: Teratoma fetiforme breve nota su di un caso. Chir Pat Sper 178-81, 1970.
  68. Yasuda Y, Mitomori T, Matsuura A et al: Fetus-in-fetu: report of a case. Teratology. 31: 337-44, 1985.
  69. Willis RA: Pathology of tumors. St Louis, Mo, CV Mosby, p 941, 1948.
  70. Sadler TW: Langman"s Medical embryology. 5th Ed. Baltimore. Williams & Wilkins p171, 1985.
  71. Hawkins EP: Fetus-in-fetu or not. J Pediatr Surg. 25: 583-4, 1990.
  72. Nochomovitz LE, Rosai J: Current concepts on the histogenesis, pathology and immunochemistry of germ cells tumors of the testis. Pathol Ann 13:327-62, 1978.
  73. Brunkow CW: Pediatric gynecology. Schauffler Chicago p220-6, 1942.

Discussion Board

Start a discussion about this article
Add bookmark Bookmarked