The condition, known as fetus in fetu, is extremely rare and is said to occur in one in 500,000 births across the globe.
Medical professionals were under the assumption that the infant, born in November 2010, had two tumours in her abdomen so surgery was carried out to remove the masses.
Upon operating on the three-week-old infant, doctors soon realised that these supposed 'tumours' contained "two fetiform structures with spine and long bone formation."
Explaining the rare condition, Dr. Yu Kai-man told The South China Morning Post: "Since it is impossible for the little girl to have conceived the pregnancy on her own, the fertilisations of the twin foetuses, of course, belongs to her parents, which has gone to the wrong place."
According to the doctor, it is impossible to detect the condition during postnatal check-ups as the embryo is too small.
