The foetus, which had developed upper limbs, bones and even fingernails, was around four inches long and was only discovered when the parents took their daughter for hospital scans because she had an enlarged head and problems with motor skills.
Foetus-in-foetu is the term used to describe this rare occurrence when twins fuse together in the womb, of which there have been only 200 documented cases with 18 happening inside the skull.
It is thought to be caused by incomplete separation when a single egg splits or as late cell division.
Despite surviving a year after birth thanks to its shared blood supply, it’s unclear if the surviving twin will suffer long-term damage.
Dr Zongze Li from Huashan Hospital, Fudan University who treated the girl said that “the intracranial foetus-in-foetu is proposed to arise from unseparated blastocysts.”
There are further similar cases in medical literature; in 2017 Thailand discovered three siblings inside the skull of an unborn girl connected by a single artery and vein; likewise in 2015, China found an unborn foetus inside its male twin’s scrotal sac.
All were successfully removed via surgery with full recovery for their twins.
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