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 Table of Contents  
Year : 2022  |  Volume : 2  |  Issue : 1  |  Page : 29-31

Fetus in fetu: A rare differential diagnosis for an antenatally identified ultrasonographic intra-abdominal mass

1 Department of Pediatric Surgery, Government Medical College, Kottayam, Kerala, India
2 Department of Pathology, Government Medical College, Kottayam, Kerala, India
3 Department of Radiodiagnosis, Government Medical College, Kottayam, Kerala, India

Date of Submission19-Sep-2021
Date of Decision07-Jan-2022
Date of Acceptance22-Jan-2022
Date of Web Publication25-Feb-2022

Correspondence Address:
Dr. Shirbin Joe Mathews
Thadathilanickal House, My Own Colony, Erayilkadavu, Kottayam - 686 001, Kerala
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ipcares.ipcares_282_21

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Background: Fetus in fetu (FIF) is a rare congenital anomaly in which a parasitic fetus is trapped inside the body of its twin. The incidence is 1 in 500,000 births. Initially, considered as a mature teratoma, it was later identified as a separate entity due to the presence of an axial skeleton and organized limbs. Clinical Description: A 48-day-old girl presented with an antenatally detected calcified intra-abdominal mass. She had a palpable retroperitoneal mass measuring 6 cm × 4 cm at the left lumbar area. Sonology showed a heteroechoic cystic mass in the left lumbar region. Serum markers were normal except for a slightly elevated serum lactate dehydrogenase. Contrast-enhanced computerized tomography of the abdomen showed a well-defined cystic lesion measuring 7.6 cm × 6.5 cm × 5.9 cm with enhancing septae, multiple calcific foci, teeth, and bones in the left suprarenal region displacing the left kidney. Management: Laparotomy showed a cystic appearing mass with solid components within. Cystic structure was the amniotic covering and solid component turned out to be the FIF with a face, limbs, and umbilical cord. No adjacent infiltration was seen. Pathology confirmed the finding of FIF with an identifiable vertebral column, umbilical cord, pelvis and lower limb bones, skin, and retinal and brain tissue. The infant has been under follow-up for a year and is thriving. Conclusion: FIF is a rare condition of infancy which can be diagnosed preoperatively by radiological examination, treated by complete excision, and confirmed by gross and histopathology. Differentiation of FIF from teratoma is mandatory as the latter can be malignant.

Keywords: Fetus in fetu, retroperitoneal calcified tumor, teratoma

How to cite this article:
Kazia AP, Mathews SJ, Vasu PP, Krishnadas S. Fetus in fetu: A rare differential diagnosis for an antenatally identified ultrasonographic intra-abdominal mass. Indian Pediatr Case Rep 2022;2:29-31

How to cite this URL:
Kazia AP, Mathews SJ, Vasu PP, Krishnadas S. Fetus in fetu: A rare differential diagnosis for an antenatally identified ultrasonographic intra-abdominal mass. Indian Pediatr Case Rep [serial online] 2022 [cited 2022 May 27];2:29-31. Available from: http://www.ipcares.org/text.asp?2022/2/1/29/338484

Fetus in fetu (FIF) is defined as a rare congenital anomaly in which a malformed parasitic fetus is trapped inside the body of its twin.[1] The incidence is reported to be 1 in 500,000 births.[2] An early description of this condition was given by Young in 1809.[3] There are two main theories regarding the etiopathogenesis of FIF. The first, “the included twin” theory proposes that it is associated with abnormal embryogenesis in a diamniotic, monochorionic pregnancy, wherein a vertebrate fetus is enclosed within the body of another normally developing fetus.[3] The second, “the teratoma” theory suggests that the FIF mass represents a well-differentiated, highly organized teratoma.[3] The FIF complex is characterized by a fibrous membrane (representing the chorioamnion) that contains some fluids (amniotic fluid) and a fetus suspended by a cord or pedicle.[4] The growth of an FIF initially parallels that of its twin but stops abruptly due to two reasons, either the vascular dominance of the host twin or an inherent defect in the parasitic twin.[5]

We present this case to highlight the multidisciplinary collaboration that resulted in favorable outcome in an infant presenting with an antenatal radiological abnormality, the need to differentiate FIF from a teratoma, and the importance of close follow-up to ameliorate the risk of recurrence.

  Clinical Description Top

A 48-day-old female infant was brought by her parents on a follow-up visit in relation to an antenatally detected abnormality detected on the ultrasound at 35 weeks gestation in the form of a left suprarenal calcification. The couple had been advised follow-up after delivery by the obstetrician and neonatologist team. Review of her medical records revealed that the mother had gestational diabetes mellitus, for which she was prescribed metformin. The baby was born at term gestation by cesarean section (CS), the indication being a previous CS. Her birth weight was 2.48 kg and the APGAR scores were normal. The perinatal period was uneventful, except for a single episode of asymptomatic hypoglycemia, the details of which were unavailable. She was the second child of a nonconsanguineous union. There was no family history of multiple pregnancies. At birth, the general physical and systemic examination had been normal. There was no evidence of any palpable abdominal mass. The baby had been discharged with advice to get a postnatal abdominal ultrasound. Because of the pandemic and since the exclusively breast fed infant was thriving well, they failed to report back until 6 weeks of age. There was no history of abdominal distention, vomiting, constipation, or perception of an abdominal mass during handling.

On examination, the baby was hemodynamically stable; heart rate 108 beats per minute, blood pressure 64/48 mm of Hg, and oxygen saturation in room air 99%. The anthropometric measurements were appropriate for the age with length 52 cm, weight 4.5 kg, and head circumference 37 cm. Abdominal examination revealed a mass measuring approximately 6 cm × 4 cm involving the left hypochondrium and lumbar region. The mass was nontender with smooth surface, firm consistency, all borders palpable, except the lateral border. There was no intrinsic mobility, and the mass displayed minimal movement with respiration. There was impaired resonance on percussion, and no bruit could be heard on auscultation. Remaining systemic examination was normal. The clinical differential diagnosis considered included retroperitoneal teratoma (antenatal calcification, well-preserved child with a smooth retroperitoneal mass, and no clinical evidence of metastasis), and neuroblastoma (presentation in early infancy, antenatal calcification, and suprarenal location).

The abdominal ultrasound showed a left suprarenal hetero-echoic cystic mass measuring 7 cm × 4 cm with areas of calcification and internal vascularity. Tumor markers were planned in view of the differentials. Most were normal; serum alpha fetoprotein (AFP) 685.7 ng/ml, 24-h urinary vanillyl mandelic acid less than 1 mg/24 h, serum beta human chorionic gonadotropin less than 1.2 IU/L. The only abnormality detected was an elevated serum lactate dehydrogenase (LDH) of 654 U/L (normal values 170–580 U/L). A contrast-enhanced computed tomography (CECT) of the abdomen was performed for more refined delineation of the mass and assessment of organ of origin. This revealed a well-defined predominantly cystic lesion measuring 7.6 cm × 6.5 cm × 5.9 cm with smooth borders, enhancing septae, multiple calcific foci, teeth, bones, and irregular fat deposits in the left suprarenal region that was displacing the left kidney posteroinferiorly [Figure 1]a. The three-dimensional reconstruction showed a fetus-shaped mass in the left hypochondrium [Figure 1]b. Based on these radiological findings, our differential diagnoses narrowed down to a well-differentiated teratoma or a FIF.
Figure 1: (a) Contrast-enhanced computed tomography of the abdomen showing a left sided mass with multiple calcifications, teeth and bones. (b) Three dimensional reconstruction of the abdominal contrast-enhanced computed tomography showing a fetus shaped mass in the left suprarenal area

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  Management and Outcome Top

We proceeded with a laparotomy with the plan to excise the mass and to send for a biopsy. Intraoperatively, the mass was located in the left lumbar area in the retroperitoneum. It was a predominantly cystic appearing mass with solid components within, having no infiltration to adjacent structures and a prominent feeding vessel to the aorta. The fluid within the amniotic covering formed the cystic part, and the FIF formed the solid part with a face, four limbs, and an umbilical cord [Figure 2]. Gross pathology showed a fetus-shaped mass with gestation age of 13 weeks, weight of 115 g, and crown rump length of 7 cm. It exhibited anencephaly, sirenomelia type 1 (Mermaid syndrome with fused lower limbs), polydactyly of the left foot, and syndactyly of both hands and feet. The thoracic and abdominal organs, anal opening and external genitalia were not developed. The FIF showed evidence of a dorsal vertebral column [Figure 3] and umbilical cord of normal morphology, with skin, pelvic and lower limb bones (femur, tibia, and fibula) on both sides. Histopathology showed retinal tissue and parts of the brain tissue (hind brain). Section from ilium and vertebrae showed hematopoietic bone marrow as well as skeletal muscle attachment.
Figure 2: Postoperative image of the fetus in fetu (*) with an umbilical cord and amniotic sac (**)

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Figure 3: Internal morphology of the fetus in fetu displaying the vertebral column (*)

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Based on the fetus-like shape and the presence of vertebral column, umbilical cord, and amniotic coverings, the final diagnosis of FIF was confirmed. The infant recovered well after surgery and was discharged on the 7th postoperative day. As per the standard protocol of FIF, she was kept on close follow-up with serial measurements of AFP and repeated ultrasounds of the abdomen for the detection of recurrence. At present, the patient is a 1-year-old with normal serum AFP (5 ng/ml) and LDH (155 U/L) and normal ultrasonography.

  Discussion Top

FIF is a rare disease that usually presents before 18 months of age.[3] There are reports of later presentation in childhood or rarely in adult life. Gender predilection is uncertain; Patankar et al.[5] noted a 2:1 male predominance, whereas Thakral et al.[6] reported equal prevalence. The most common presentation of FIF is a mass, most commonly in the abdomen, and almost 80% in the retroperitoneum.[7] The FIF may be asymptomatic or produce symptoms due to a mass effect such as abdominal distention, difficulty in feeding, vomiting, jaundice, and pressure effects on the renal and/or respiratory systems.

The differential diagnosis of FIF includes causes of a neonatal intra-abdominal mass with calcification such as meconium peritonitis, neuroblastoma, adrenal hemorrhage, and a retroperitoneal teratoma. The first line of investigation is radiological. Ultrasonography shows a complex cystic mass with ill-defined solid internal components, which helps differentiate from meconium peritonitis, adrenal hemorrhage, and solid tumors. However, excluding a teratoma may need more sophisticated modalities such as CECT and magnetic resonance imaging by demonstration of the axial skeleton. Sometimes, it may not be possible to clearly differentiate FIF from a teratoma without histopathology.

Hence, the definitive treatment of suspected FIF is surgical excision. Since there are many similarities between FIF and teratoma at the histological level, a detailed pathological examination is imperative. An important feature that distinguishes FIF from a teratoma is the presence of a vertebral column.[6] Its presence shows that fetal development has advanced at least beyond the primitive streak stage (12–15 days of gestation) to a notochord, which is the precursor of the vertebral column.[3],[6] Other characteristics of an FIF include a mass enclosed within a distinct sac, partially or completely covered by skin, grossly recognizable anatomic features such as limbs, and attached to the host by a pedicle containing relatively large blood vessels, as was seen in this case.[3],[6]

FIF is almost always benign. Only one malignant case has been reported.[8] The mass recurred as a yolk sac tumor 4 months after removal which was attributed to the presence of immature tissues and remnants of the capsule of the mass. Thus, it is recommended that the FIF mass should be excised in toto including its coverings to prevent local recurrence.[8] Serological (AFP) as well as radiological follow-up is required[2] for early detection of immature elements, especially when a part of the sac has to be left in situ due to its proximity to major blood vessels.

In conclusion, FIF is considered as a benign condition whereas teratoma which is a close differential diagnosis is a potentially malignant diagnosis. In this case, close collaboration between the obstetrician, neonatologist, radiologist, surgeon, and pathologist resulted in complete workup, complete surgical excision, confirmation of diagnosis, and favorable outcome in an infant who presented with an antenatally detected radiological abnormality.

Declaration of patient consent

The authors certify that they have obtained the appropriate consent from the legal guardian. In the form, the legal guardian has given his consent for the images and other clinical information to be reported in the journal. The guardian understands that the name and initials will not be published, and due efforts have been made to conceal the same, but anonymity cannot be guaranteed.

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Conflicts of interest

There are no conflicts of interest.

  References Top

Karaman I, Erdoğan D, Ozalevli S, et al. Fetus in fetu: A report of two cases. J Indian Assoc Pediatr Surg 2008;13:30-2.  Back to cited text no. 1
[PUBMED]  [Full text]  
Hoeffel CC, Nguyen KQ, Phan HT, et al. Fetus in fetu: A case report and literature review. Pediatrics 2000;105:1335-44.  Back to cited text no. 2
Young GW. Case of a fœtus found in the abdomen of a boy. Med Chir Trans 1809;1:236-64.  Back to cited text no. 3
Gangopadhyay AN, Srivastava A, Srivastava P, et al. Twin fetus in fetu in a child: A case report and review of the literature. J Med Case Rep 2010;4:96.  Back to cited text no. 4
Patankar T, Fatterpekar GM, Prasad S, et al. Fetus in fetu: CT appearance-report of two cases. Radiology 2000;214:735-7.  Back to cited text no. 5
Thakral CL, Maji DC, Sajwani MJ. Fetus-in-fetu: A case report and review of the literature. J Pediatr Surg 1998;33:1432-4.  Back to cited text no. 6
Willis RA. The Borderland of Embryology and Pathology. 2nd ed. London: Butterworth & Co. (Publishers) Ltd.; 1962. p. 442-62.  Back to cited text no. 7
Hopkins KL, Dickson PK, Ball TI, et al. Fetus-in-fetu with malignant recurrence. J Pediatr Surg 1997;32:1476-9.  Back to cited text no. 8


  [Figure 1], [Figure 2], [Figure 3]


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