Fact Check: Why some babies are born with a twin stuck inside

There is still an element of debate on whether a FIF mass is really a parasitic twin or just a tumour made up of different kinds of cells that is so well organised that it gives the impression of a malformed human foetus.

In February, a baby was delivered by doctors in Colombia with her twin stuck inside her. The baby, who was operated on immediately to remove her partly formed twin, is now doing well.

This has brought into focus a little known condition, foetus in foetu (FIF) , in which twins are formed post fertilisation but, because of a quirk of nature, one twin becomes a "parasite" of the other – it starts growing inside the body of its twin.

While FIF is unusual, and very few cases have been documented in literature, it is not extremely rare - it is estimated at one in every 5 lakh births. In February 2018, The Indian Express reported a successful surgery in Ahmedabad that removed a malformed foetus from a seven-month-old boy. In 2017, Malaysian doctors reported in The BMJ the case of a 15-year-old boy who had an abdominal mass since infancy that was later found to be a case of FIF.

There is still an element of debate on whether a FIF mass is really a parasitic twin or just a tumour made up of different kinds of cells that is so well organised that it gives the impression of a malformed human foetus. It is usually a benign condition, which is why babies often survive into adolescence with the mass still inside.

FIF is most commonly found in the abdomen, but there have been some cases where it was located in the head, sacrum, scrotum or mouth.

"The term Fetus in fetu was coined by (German anatomist Johann Friedrich) Meckel during the late 18th century… FIF usually occurs as a single lesion however multiple FIF have also been reported, highest being 5. FIF is always a curiosity and to date about 200 cases have been reported in literature," Pakistani researchers reported in the APSP Journal of Case Reports in 2012.