The fetus of a twin has been surgically removed from the skull of its one-year-old sister — in a medical anomaly only ever recorded a handful of times.
Doctors said the fetus had developed upper limbs, bones and even fingernails, meaning it likely continued growing for months while inside its sibling in the womb.
The fetus – which was about four inches long – was only discovered when the parents took their daughter for hospital scans because she had an enlarged head and problems with motor skills.
Fetus-in-fetu is the medical term for the rare phenomena that sees twins fuse together in the womb and one develops physically inside another.
Only around 200 cases have ever been documented, of which just 18 occurred inside the skull.
Fetus-in-fetu has also been detected in the pelvis, mouth, intestines and even the scrotum.
The condition is caused by the incomplete separation of identical twins, who form when one egg splits. Doctors don’t know exactly how this happens.
Some have theorized that the healthy twin connects to the mother via the placenta, while the other latches onto the twin’s blood vessels.
As the bigger twin grows, the smaller one becomes absorbed into their abdomen. Other scientists have suggested that it happens as a result of late cell division.
The unviable fetus may continue to develop for several weeks and months inside its sibling — even forming organs, bones and limbs.
The latest tale was revealed in December in the American Academy of Neurology’s journal Neurology.
The unnamed girl was taken to hospital after showing problems with her motor skills.
Doctors did not give any further details, but this may include problems with her walking or sitting.
CT scans revealed her unborn sibling was pressed against her brain.
She also had hydrocephalus, the build-up of fluid deep within the brain that can cause an enlarged head, extreme sleepiness and seizures.
Doctors said it had continued to survive a year after birth because it shared a blood supply with its sibling.
It was unclear if the surviving twin will suffer long-term damage.
Dr Zongze Li, a neurologist at Huashan Hospial, Fudan University who treated the girl, said: ‘The intracranial fetus-in-fetu is proposed to arise from unseparated blastocysts.
‘The conjoined parts develop into the forebrain of the host fetus and envelop the other embryo during neural plate folding.’
The case is one of only 18 reported in medical literature to date.
Doctors in Thailand in 2017, found three siblings inside the skull of an unborn girl.
They said each had ‘multiple well-developed organs’, including a nervous, digestive and respiratory system.
They were connected to the host sibling via a single artery and vein, which the doctors said had been the umbilical cord.
In another case from 2015, also in China, doctors found an unborn fetus inside the scrotal sac of its male twin.
The 20-day-old infant was taken to hospital after birth when his scrotum started to swell up.
Scans revealed a ‘well-defined… mass’ within the scrotum, complete with bones and buds that doctors said would have formed into limbs.
The fetus was removed via surgery and its twin was discharged five days after surgery, having made a full recovery.
What is fetus-in-fetu?
Fetus-in-fetu (FIF) occurs when a malformed fetus is enclosed within the body of its twin.
It is thought to occur in around one in every 500,000 births and can affect both males and females.
Fewer than 200 cases of the extremely rare condition have been reported in medical literature. There are only 18 known where the fetus has been spotted inside the skull of its sibling.
It is usually spotted in infancy but a handful of cases have been reported among adults.
Symptoms include a large abdominal mass. Scans are needed to confirm that a FIF is present.
Usually, one to three FIF are present. But in one case in India in October 2022, eight fetuses were found.
Doctors can then remove the fetus through surgery. Sufferers usually make a full recovery.