Explained: What is foetus in foetu?

When a ‘twin’ forms inside a human body. What causes it, and how frequently does it happen?

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Foetus in Foetu is a condition in which a malformed parasitic foetus is usually found in the abdominal cavity of the living twin (host).

When a 17-year-old girl Indian girl recently went to get a “lump” in her abdomen treated, it was found to be her twin growing inside her. This is a condition called foetus in foetu (FIF), or foetus within a foetus. Doctors say this is the first reported case of FIF in an adult woman worldwide, and the eighth such case in an adult in India (the previous cases were in adult men).

In this condition, a malformed parasitic foetus is usually found in the abdominal cavity of the living twin (host). Fewer than 200 cases of this condition have been reported in medical literature and it occurs in about 1 in 5,00,000 live births. While FIF can occur in various age groups, it is most commonly diagnosed in patients younger than 18 months.

In the case of the Indian 17-year-old, her abdomen would often hurt and she would feel full. However, no history of weight loss or altered bowel movements and urinary symptoms were recorded. Her “menstrual habits” too were within the normal limits, according to BMJ Journals where her case has been published. When the lump was microscopically examined, it measured 30×16×10 cm and was found to be composed of a “hairy cheesy material, multiple teeth and structures resembling limb buds.”

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According to BMJ Journals, there are two theories about the origins of FIF. The first is where a parasitic twin’s malformed foetus is formed inside the body of the host twin and both share the blood supply.

The other is that FIF is a “highly differentiated” form of teratoma – tumours made from tissues foreign to the area or part of the body in which they are found.

What differentiates a FIF from a tumour is that the lumps in the case of FIF are benign (do not spread to other organs and tissues) in nature, and are of embryological origin.

When was the expression “foetus in foetu” first used? According to the BMC Journal of Medical Case Reports, it was first used by Johann Friedrich Meckel the Younger in the late 18th century. In a paper titled “Fetus in Fetu and the Retroperitoneal Teratoma” and published in 1960, R H Lewis credits Meckel with the use of the term around the 1800s.

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One of the early cases of the condition surfaced in a patient called John Hare, born on May 18, 1807. The abnormality in the baby’s abdomen led him to lose weight rapidly, “until at length he was nearly 36 inch in circumference”, and he eventually died.