Fetoception: fetus within a fetus: a rare case presentation in a neonate

INTRODUCTION

Fetus in fetu (FIF) is a rare anomaly of embryogenesis in which a malformed parasitic twin is found within the body of the normally developed host.¹ The majority of cases appear in infancy with an incidence reported at one in 500,000 births.² The FIF is quite similar to a mature teratoma; however, a key difference is that a mature teratoma has an independent growth ability and malignant potential.³

Histopathologic findings confirm the diagnosis, and the recommended treatment is complete excision in both cases. We report one case of an FIF in a neonate, located at sacrococcygeal region mimicking a large mature teratoma.

CASE REPORT

A female baby born at 37 weeks of gestation, weighing 3.335 kg, had a large mass over sacrococcygeal region. The mass was a large sac with a small body coming out of it with malformed limbs with fingers (Figure 1). Mother was a second gravida with a healthy living first born. Antenatal history was not significant except for anomaly scan which was s/o a large sacral mass. No family history of any congenital anomaly was present. The newborn was otherwise vitally and hemodynamically stable with all systems functionally normal with normal movements of both lower limbs and normal reflexes. No urinary or defecation issues noted. Plain radiograph was suggestive of presence of two vertebral bodies in the mass (Figure 2). Magnetic resonance imaging (MRI) of the region was done with computed tomography (CT) angiography which was suggestive of a teratoma-like large soft tissue lesion (Figure 3). The baby was operated on day four of life and complete resection of the mass was done.

Pathological examination revealed that the mass was partially covered with skin, measuring 18.0×14.0×7.8 cm, and weighing 870 grams, including a malformed trunk with four limbs of which one limb had eight digits while the rest of the three limbs had four digits each. No structure resembling a head was noted on examination. The vertical cut section revealed fatty tissue along with coiled intestinal loops, stomach as well as bones and cartilage. Multiple sections studied microscopically from different sites revealed well organized fetal tissues comprising of skin with adnexal structures, adipose tissue, cartilage, blood vessels, striated muscle, adrenal gland, salivary glandular elements, intestine, well organized neural tissue as well as lymphoid tissue along with bone and cartilage. There was no evidence of atypia or malignancy. Baby was well throughout the post-operative stay and was discharged at day 11^th of life (Figure 3 & 4).

Management and outcome

FIF is a rare and challenging condition that requires prompt diagnosis and intervention. In this case, a neonate with a sacrococcygeal mass underwent a detailed diagnostic workup, including radiographs, MRI, and CT angiography, which initially suggested a teratoma-like lesion. However, the final diagnosis of FIF was made following pathological examination, which revealed well-organized fetal tissues such as limbs, intestinal loops, and vertebrae, confirming the presence of a parasitic twin.

The management of FIF primarily involves complete surgical excision of the mass. In this case, the infant underwent surgery on day four of life, with the mass being successfully removed. The operation was uneventful, and the neonate had a stable post-operative recovery. The absence of malignancy in the pathological findings further supported the diagnosis of FIF, distinguishing it from mature teratomas, which can carry a risk of malignancy.

Postoperatively, the baby remained vitally stable, showing no complications, and was discharged on the 11th day of life, reflecting a positive outcome. No issues were noted with urinary or gastrointestinal functions, and the baby displayed normal reflexes and limb movements, which further supported the absence of any significant neurological deficits or other congenital anomalies.

This case emphasizes the importance of early and accurate diagnosis, distinguishing FIF from other teratomatous lesions, and underscores the necessity of surgical resection as the primary treatment modality. The positive outcome highlights the effectiveness of early intervention, ensuring the infant's full recovery and preventing potential complications. Further studies are required to refine diagnostic criteria and improve differentiation between FIF and other similar conditions, such as mature teratomas, to provide better clinical management and outcomes for affected individuals.

DISCUSSION

FIF, first described by Johann Friedrich Meckel, is the malformed or parasitic monozygotic diamniotic twin that is found inside the body of a living child or sometimes in an adult. The pathogenesis is still largely unknown; but during the ventral folding of trilaminar embryonic cyst (second and third weeks of development), the diamniotic monochorionic twin is included within its host.⁴ The inclusion of FIF in the sister embryo is speculated to be because of a persistent anastomosis of the vitelline circulation of the feti during their development.⁵ Another hypothesis suggest that it results from unequal division of the totipotent inner cell mass of the developing blastocyst, causing a small cell mass within a maturing sister embryo.⁶ This ultimately results in a vestigial remnant, or FIF.

Clinically, FIF can be differentiated from teratoma by the presence of vertebral bodies and limbs. The presence of vertebral bodies not only means that the FIF has passed the primary stage of gastrulation but may also reflect its derivation from a primitive streak during third week of gestation. Primitive streak further leads to the notochord formation and subsequently to the vertebral column and segmental axis. Therefore, the fetiform mass develops to a certain degree in a manner similar to normal fetal development.⁷ In contrast, teratoma consists of pluripotent cells, without organogenesis or vertebral segmentation.⁸ Multiple existing literature indicate that teratoma and conjoined twins appear to be two ends of a spectrum of monozygotic twinning with FIF lying in between with overlapping manifestations.⁹

Gonzalez–Crussi proposed definition of FIF that, FIF is applied to any structure in which the fetal form is in a high stage of organogenesis and the vertebral axis is present. This criteria is often followed to diagnose FIF.¹⁰ Spencer has proposed another criteria for labelling any fetiform mass as FIF with presence of one or more of the following criteria: mass needs to be enclosed within a distinct sac, should be partially or completely covered by normal skin, should have grossly recognizable anatomic parts, should be attached to host by only few relatively large blood vessels, and either be located immediately adjacent to one of the sites of attachments of conjoint twins or associated with neural tube or the gastrointestinal tract.³ But this criteria do not mention presence of vertebra as an essential criteria for labelling FIF though.

CONCLUSION

The case discussed in our report fulfils both the criteria to be a FIF especially in view of presence of developed organs and vertebrae, though further definitive research studies should help us to differentiate FIF from a mature teratoma, which carries a 10% malignant potential.

Recommendations

Rare congenital anomaly: FIF is a rare anomaly often confused with mature teratomas but is distinguished by the presence of organized fetal structures and vertebrae.

Clinical and diagnostic challenges: Differentiating FIF from a mature teratoma requires thorough imaging and pathological examination, as both conditions present overlapping features.

Successful management: Complete surgical excision is the definitive treatment, as seen in this case, where the neonate recovered fully postoperatively without complications.

Funding: No funding sources

Conflict of interest: None declared

Ethical approval: Not required

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