A fetiform placental lesion in a monochorionic diamniotic placenta: a diagnostic challenge between teratoma and an acardiac co-twin—a case report

Placental teratomas are extremely rare non-trophoblastic tumours of the placenta. A fetiform teratoma is a rare subtype of mature teratoma consisting of highly differentiated tissues that may resemble a malformed fetus but lack true organogenesis, axial skeletal organisation and developed systemic anatomical structures. However, it can be difficult to distinguish fetiform-like lesions, particularly those with placental attachment, from a severely regressed or acardiac twin. This case report presents the clinical, macroscopic and histopathological findings of an unusual placental mass, resembling a fetus and containing hair-like structures, which was incidentally detected during a caesarean section in a 26-year-old patient who had had an uneventful antenatal course. Histopathological examination revealed a monochorionic diamniotic placenta containing two umbilical cords, one of which was markedly short, and an adjacent lesion measuring approximately 5 × 5 × 2.5 cm. Microscopic examination revealed mature tissues derived from multiple germ layers, including skin and skin appendages, adipose tissue, cartilage, striated muscle, ganglionic elements, central nervous system tissue and the choroid plexus. No vertebral axis, long appendicular bones, organised visceral structures or developed systemic anatomical structures were detected. However, the monochorionic diamniotic placental structure and the markedly short second umbilical cord raised the possibility of a severely regressed twin. Whilst the morphological features of the lesion are consistent with a mature fetiform teratoma, a severely regressed or acardiac monozygotic twin could not be definitively ruled out on the basis of morphological findings alone. Without molecular comparative analysis, it is difficult to make a definitive diagnostic distinction between these two possibilities. This case highlights the importance of interpreting morphological findings in conjunction with placental anatomy and, where possible, molecular data when assessing foetiform placental lesions.