In 2010, a baby girl in Hong Kong was born pregnant with twins. This abnormality, known as ‘foetus in fetu,’ is said to occur in 1 in 500,000 births and is characterised by the foetus growing inside the other foetus. As a result, the twins are born conjoined.Other cases of this illness have been reported throughout history. In 1808, a set of twins was born in France, with one of them having a tiny foetus growing inside their abdomen. Another set of twins was born in England in 1827, one of whom had a tiny foetus attached to their liver.This disorder is hypothesised to occur when a fertilised egg divides into two, but one of the twins fails to develop correctly and instead begins to grow inside the other twin. This can occur either during pregnancy or after the twins are born.This illness has no known remedy and is generally fatal for both twins. In certain circumstances, surgery to separate the foetus from the other twin is possible, however this is not always possible or successful.The twins born in Hong Kong in 2010 are thought to be the first known case of ‘foetus in fetu’ in which both twins survived.