A 17-year-old woman sought medical attention for a lump in her abdomen, which had been growing in size over the course of five years, doctors have outlined in a BMJ Case Report.
The growth was causing on-and-off pain, though otherwise, her heath was normal. On inspection, the lump was irregular and hard in consistency. Further investigations were ordered, at which point a CT scan showed a "well-defined mass" that was displacing the organs in her abdomen. From the scan, doctors could see the mass had "fat density areas, soft tissue and multiple calcified density components of various sizes and shapes resembling the shape of vertebrae, ribs and long bones."
From this, she was diagnosed with "fetus in fetu" (FIF), an extremely rare abnormality where a fetus becomes enveloped by its own twin. Less than 100 cases have ever been recorded worldwide, with the abnormality happening in only one per 500,000 births.
One theory of how it happens suggests that the "parasite" twin begins as a normal fetus, sharing a placenta with its sibling. Early in pregnancy, however, the host twin envelopes the other twin. The twin that has been enveloped becomes a "parasite" as it uses the host twin's blood supply in order to survive. In cases of fetus in fetu, the "parasite" twin lacks the organs it needs to survive outside of the twin and threatens the life of the host it inhabits, as both twins try to get the nutrition they need from a single umbilical cord.
Another theory is that FIF is a form of teratoma (a tumor made up of several different types of tissue, such as hair, muscle, teeth, and bone). Most cases of FIF, the authors write, are discovered in infancy, but not so in this case.
Exploration of the woman's abdomen found a large 30×16×10-centimeter (12×6×4-inch) mass under her liver, consisting of hair, "mature bones", and "other body parts", which the surgeons removed. On examination of the removed mass, the doctors found "hairy cheesy material, multiple teeth and structures resembling limb buds".
Other than this patient, from India, only seven other adult cases of FIF have been reported, with the oldest person diagnosed being 47 years old.
Following the removal of the mass, the unnamed patient is recovering well.
"I am feeling very well and my abdomen is now flat and my parents are also very happy," she told the team that wrote the report. "Thanks to all operating doctors."