Doctors were left stunned when a baby girl was born 'pregnant' with twins.
According to reports in Hong Kong, the baby was found to have two foetuses growing inside her and an operation was carried out to remove them when she was just three weeks old.
The foetuses, which were covered in skin, were eight to 10 weeks gestated and had legs, arms, a spine, rib cage and intestines.
One weighed 14.2g and the other 9.3g - and each had an umbilical cord.
The surgery was successful and the girl was discharged from the Queen Elizabeth Hospital in Kowloon eight days later.
The condition, known as foetus in foetu, occurs in just one in 500,000 births in the world. Fewer than 200 cases have ever been reported.
Dr Yu Kai-man, a specialist in obstetrics and gynaecology, told the Hong Kong Medical Journal: "It was almost impossible to detect during the prenatal check-up as the embryo inside the baby was too small."
Yu, a former professor at the Chinese University of Hong Kong, added: "Since it is impossible for the little girl to have conceived the pregnancy on her own, the fertilisation of the twin foetuses, of course, belongs to her parents, which has gone to the wrong place."
The report said the reason behind the abnormality, which happened when the girl was born in 2010, is still unknown and the World Health Organisation classified it as a type of cancer. But it suggested it was possibly linked to the mother having multiple abortions.
The report said: "The widespread use of antenatal ultrasound in early gestation may provide more concrete evidence and shed light on this intriguing condition."