Having a child is a blessing; some are even fortunate to have twins or more—a true bundle of joy. But what if one twin never fully developed and instead trapped inside the other? Sounds like fiction, right? This unusual phenomenon is called fetus-in-fetu, one of the rarest medical cases known to humankind.
Unimaginable as it seems, Fetus-in-fetu (FIF) is a rare congenital anomaly in which a fetus grows inside the body of its twin. Known as the 'parasitic twin,' it typically appears as a lump or mass inside the body and can remain undetected for the first few months after birth [1, 2]. But how does this condition occur?
With less than 200 reported cases worldwide, FIF occurs due to the incomplete development of one of the identical twins during the early stages of embryogenesis, a process of cell division and growth forming an embryo [2, 3]. The parasitic twin also lacks an amniotic sac, which would normally support its development. This leads to it being dormant and absorbed by its twin's body instead of becoming a fully developed fetus. FIF is commonly found in the abdomen, while other cases have been observed in the pelvis, tailbone, skull, oral and thoracic cavities, and scrotum [4, 5]. Fortunately, the 'host' twin typically survives, with the 'parasitic' twin being removed through surgery [1].
The first case of FIF was recorded during the 18th century by Johann Friedrich Meckel, a German anatomist (1781-1833) [6]. Meckel's documentation of this medical case opened the door to understanding this parasitic twin phenomenon, which remained unexplained for centuries, until recently gaining an in-depth scientific explanation.
References
Sharma, A., Goyal, A. & Sharma, S. (2012). Fetus in fetu: A rare case report. Journal of Research in Medical Science. 17(5): 491-494. PMID: 23626618; PMCID: PMC3634279
Schnirman, R., Kuo, S., Kelly, R. & Yamaguchi, T. (2023). Chapter five: The role of Wnt signaling in the development of the epiblast and axial progenitors. Current Topics in Developmental Biology. 153: 145-180. doi: 10.1016/bs.ctdb.2023.01.010
Agarwal, A., Agrawal, M. & Ahuja, S. (2024). Fetus in fetu in a 1-year-old female: A case report. Journal of Pediatric Surgery Case Reports. 106: 1-6. doi: 10.1016/j.epsc.2024.1022830
Karaman, I., Erdogan, D., Ozalevli, S., Karaman, A., Cavusoglu, H., Aslan, K. & Cakmak, O. (2008). Journal of Indian Association of Pediatric Surgeons. PMCID: PMC2810823. PMID: 20177485
Abdulraheem, N., Nasir, A., Abdur-Rahman, L., Akanbi, O., Olanrewaju, M., Alada, M., Oyedepo, O., Obasa, T. & Adeniran, J. (2015). Oral fetus-in-fetu: A case report. Journal of Pediatric Surgery Case Reports. 3(4): 171-173. doi: 10.1016/j.epsc.2015.02.007
Stevanny, B.., Mirani, P., Thene, E. & Kesty, C. (2024). Prenatal diagnosis of well-developed fetus in fetu with spontaneous movement in a resource-limited setting: A case report. Case Reports in Women's Health. 41: 1-4. doi: 10.1016/j.crwh.2024.e00581 Assessed and Endorsed by the MedReport Medical Review Board