Genetically Proven Retroperitoneal Fetus-in-Fetu Successfully Excised in a Neonate
Celia Permuy1
, Mercedes Sampedro1, María Sánchez-Holgado1, Elena Mansilla2, Carla Ramírez3, Saturnino Barrena3, Adelina Pellicer4
1La Paz University Hospital, Neonatology, Madrid, Spain
2La Paz University Hospital, Genetics, Madrid, Spain
3La Paz University Hospital, Paediatric Surgery, Madrid, Spain
4La Paz University Hospital, Chair of Neonatology Department, La Paz Hospital Institute for Health Research (IdiPAZ Foundation), Madrid, Spain
Keywords: Newborn, Genetic Testing, Fetus Abnormalities, Retroperitoneal Neoplasms, Monozygotic twinning
Abstract
Background: Fetus in fetu (FIF) is an exceptionally rare congenital anomaly, often difficult to distinguish from fetiform teratoma. We report a genetically proven retroperitoneal FIF in a neonate and review the literature.
Case Presentation: A term female infant presented with a prenatal retroperitoneal cystic mass. Postnatal imaging demonstrated calcifications suggestive of vertebral structures, although teratoma remained in the differential diagnosis. Serum tumor markers showed markedly elevated alpha-fetoprotein (AFP) and normal beta-human chorionic gonadotropin (β-hCG). Complete excision at 2 months revealed a sac containing a fetiform mass with scalp. Histology identified only mature tissues (glial tissue, choroid plexus, retinal pigment epithelium, dental alveoli, lamellar bone with hematopoiesis), without immature elements. Genetic testing by quantitative fluorescent PCR confirmed a diallelic profile identical to the host (46,XX), establishing the diagnosis of FIF. AFP normalized within 6 months, and the infant remained asymptomatic at 18 months.
Discussion: Imaging, particularly CT and MRI, is essential for diagnosis and surgical planning, though distinction from teratoma can remain uncertain. Histology may suggest FIF but is not always definitive. Genetic identity between host and mass provides diagnostic certainty, resolving the long-standing controversy with fetiform teratoma. Our review confirms approximately 250 cases reported to date, with ~80% retroperitoneal and excellent outcomes following complete resection.
Conclusion: FIF should be considered in neonates with retroperitoneal masses. Genetic analysis is crucial when pathology is inconclusive, ensuring diagnostic accuracy and guiding follow-up.
Cite this article as: Permuy C, Sampedro M, Sánchez-Holgado M, Mansilla E, Ramírez C, Barrena S, Pellicer A. Genetically proven retroperitoneal fetus-in-fetu successfully excised in a neonate. Pediatr Acad Case Rep. 2026;5(2):46-51.