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Articles » Multiple gestations » Fetus in fetu

1993-11-01-21 Fetus in fetu © Hill

Fetus in fetu

Lyndon M. Hill*, MD, Sandy Kislak, RT, RDMS, Mamdouha Ahdab-Barmada, MD

*Magee-Womens Hospital, Ultrasound Department, 300 Halket Street, Pittsburgh, PA 15213 Ph: 412-641-1959 Fax 412-641- 1079. §Children"s Hospital of Pittsburg.

MESH Monsters-pathology, -etiology; Fetus-pathology; Teratoma-diagnosis; Diseases-in-Twins; ICD9 651-8 CDC 651.800


Fetus-in-fetu is a rare congenital abnormality that is believed to originate from the inclusion of a parasitic twin within its partner. Most cases that have been reported in the literature occur within the upper retroperitoneum or abdomen1. Unusual locations for fetus in fetu have included the scrotal sac2, pelvis3 and iliac mesentery4.

The relationship between fetus-in-fetu and a highly organized teratoma remains controversial. Teratomas usually show a tendency toward progressive uncoordinated growth. Traditionally, teratomas are differentiated from fetus in fetu by the lack of a notochord or vertebral axis5. More recently, such criteria have been questioned6,7.

We wish to report a case in which the antenatal detection of a T3-T12 spinal gibbosity without well-defined spinal axis was diagnosed as fetus in fetu at autopsy. However, the position of the mass and the absence of most visceral organs suggested that it may actually be a well-differentiated teratoma. Fetus in fetu and teratoma may be part of a continuum with no clear line of demarcation between the two pathological processes. Both malformations are expressions of complex early embryonal dysgenesis.

Case report

The patient is a 22-year-old G2 P1 white woman who was first seen at 20 weeks" gestation. An ultrasound examination revealed a single fetus in vertex presentation. The composite assessment of gestational age was consistent with the patient"s dating parameters. The amniotic fluid volume was within normal limits. An L2-L4 open neural tube defect was detected (fig. 1). The transverse cerebellar diameter of 17 mm was 2 SD below the mean for the patient"s gestational age. In addition, the cerebellum was deviated into the cisterna magna (Arnold-Chiari II malformation). Within the thoracic region to the left of the midline and posterior to the spine was a 15 mm irregular echogenic mass (fig. 2-3). The fetal spine was intact immediately below the mass. A urea termination of pregnancy was performed one week later.

Figure 1: Axial section of the lumbar spine. The graticules outline the meningomyelocele.

Figure 2: The dorsal spinal gibbosity is outlined by the arrows.

Autopsy findings

The presence of a Chiari malformation, type II, was confirmed. There was cervical syringomyelia and probable diplo­myelia, although maceration was severe and prevented more definitive conclusions. A meningocele was also present, separate from the dorsal gibbosity, and without associated spinal cord anomalies.

The dorsal spinal gibbosity (fig. 3) contained well-formed fetal organs that were disorderly arranged. Limbs, adrenal glands, intestinal wall and lungs were detected (fig. 5). The central nervous system was well organized; portions of brain, with ependyma and choroid plexuses, and an architecturally defined spinal cord within a spinal canal were present in this dorsal gibbosity. The apparent duplication of the spinal canal, containing neural structures, was immediately dorsal to the fetal spinal canal and focally only separated from it by dura mater and fibrous tissue without bony interposition (fig. 6). The fetal vertebral bodies were widened at this level and “butterfly-shaped" with focal absence of the dorsal spinous process (apparently forming a vertebral body to the "fetus-in fetu").  

Figure 3:   Longitudinal section of the thoracic fetal spine (sp). The dorsal spinal gibbosity is outlined by the arrows.

Figure 4: Fetus after delivery. The dorsal spinal gibbosity and lumbar meningomyelocele are evident.

Figure 5: Cross-section through the fetal cervical spinal canal (arrow-head) with a dorsal irregular mass containing lung (A), adrenal gland (B) and bowel (C). (Paraffin embedded block, Hematoxylin and Eosin stained section, mag x 2.5).

Figure 6: Cross-section of the dorsal gibbosity adjacent to the fetal spinal adjacent to the fetal spinal canal, parallel to figure 4, to illustrate the "fetus in fetu" vertebral body (arrow) and irregular lateral extensions of the lateral spinous processes. [Hematoxylin and Eosin stain, meg x 25].


Willis5 differentiated between teratomas and fetus in fetu by arbitrarily stating that the latter diagnosis required the presence of an axial skeletal system -indicating that the mass had passed through the primitive strreak stage of development. However, in some reported cases of fetus in fetus the vertebral column has been underdeveloped and difficult to identify. The presence of an amnion or umbilical cord, tissue indigenous to an embryo/fetus, is also considered acceptable diagnostic criteria for fetus-in-fetu8. Serologic assessment of one fetus-in-fetu indicated that it was a monozygous twin9.

In one case, the diagnosis of a fetus-in-fetu was made preoperatively by means of a plain abdominal radiograph10. Nocera and co-workers11 suggested that a CT scan demonstrating a tubular configuration of almost pure fat around a central body density (a leg or an arm) should suggest a diagnosis of fetus-in-fetu rather than a teratoma. To our knowledge there have only been two cases in the English literature in which a fetus in fetu was visualized with antenatal sonography12,13. In one case, a 6 cm spheroidal intra-abdominal mass was visualized in the left upper abdomen of a third trimester fetus. Since the mass contained a complex echoic pattern in which fluid, soft tissue and calcification were identified, the presumptive diagnosis was a teratoma. After delivery, a plain film of the neonatal abdomen revealed a vertebral column and a rib cage within the epigastric mass. CT examination confirmed the radiographic findings of a vertebral column. The preoperative diagnosis was therefore changed to fetus in fetu. At laparotomy, a 6 cm encapsulated mass was removed that contained an irregular fetiform mass that resembled a vertebral column, thoracic cage, clavicles, humeri, ulna and cranial vault.

Our case adds further credence to the fact that the diagnosis of a fetus in fetu may be difficult to differentiate from that of a highly organized teratoma. It is generally acknowledged that teratomas may be exceedingly complex. An intracranial teratoma has been reported with teeth, eyes, mouth, intestines and extremities14, while an ovarian teratoma has demonstrated the sequential eruption of teeth from a mandible15. Heifetz and co-workers12 have pointed out that cases of fetus in fetu behave differently from retained fetuses in abdominal pregnancies (i.e., the saponification and skeletonization of a longstanding abdominal pregnancy has not been described in cases of fetus in fetu). Finally, there have been case reports of a fetus-in-fetu and a teratoma in the same patient3,7. Both highly organized teratomas and fetus in fetu may present with different degrees of spinal dysgenesis or residual posterior enteric remnants16,17 secondary to early focal disturbance of endodermal-ectodermal differentiation18.,19.

In conclusion, fetus-in-fetu may not be a distinct entity, but actually a mature, highly organized embryonal dysgenesis, or even a teratoma that has developed into architecturally well defined and vascularized organs. It usually occurs retroperitoneally, but occasionally may occur at other sites. The antenatal sonographic detection of a fetal mass that resembles a skeleton or other fetal parts mandates a neonatal evaluation with ultrasound and CT imaging. Surgical extirpation is the treatment of choice for either a mature teratoma or the possible rare case of fetus-in-fetu.


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15. Wollin E, Ozonoff MD. Serial development of teeth in an ovarian teratoma. N Engl J Med 265:897, 1961.

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17. Bremer JL. Dorsal intestinal fistula; accessory neurenteric canal; diastematomyelia. Arch Pathol 54:132, 1952.

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19. Prop N, Frensdorf EL, Vande Stadt FR. A postvertebral entodermal cyst associated with axial deformities: a case showing the “entodermal-ectodermal adhesion syndrome”. Pediatrics 39:555, 1967

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