From observation to aetiology: a case report of a twin fetus-in-fetu and a revisit of the known rarity.

A baby girl presented with an antenatal diagnosis of a retroperitoneal tumour. Postnatal imaging suggested that this mass contained two fetiform structures with spine and long bone formation. This teratomatous mass was completely excised at 3 weeks of age. Histology was consistent with twin fetuses-in-fetu, revealing two fetiform masses each with an umbilical cord connecting to a common placenta-like mass. Despite a difference in the weight of the twin fetuses-in-fetu, the level of organogenesis was identical and corresponded to fetuses of 10 weeks of gestation. Each mass had four limbs, intact skin, rib cage, intestines, anus, ambiguous genitalia, primitive brain tissue and a spine with ganglion cells in the cord. Although considered a mature teratoma in the current World Health Organization classification, the theory of formation from multiple pregnancies has been commonly implied in more recent literature. The true aetiology of this rare condition remains unclear.