Doctors Remove Fetus Of 'Parasitic Unborn Twin' From One-Year-Old Sister's Skull

By Samantha in Health and Fitness Published On 11th March 2023

Doctors have removed the fetus of a twin that was surgically removed from the skull of its one-year-old sister — in a medical anomaly only ever recorded a handful of times.

It is reported that the fetus developed upper limbs, bones, and even fingernails which means the fetus continued to grow for months while it was inside the sibling in the womb.

The four inches long fetus was discovered when the parents took their child to the doctors for scans as she had an enlarged head and problems with motor skills.


Fetus-in-fetu is the medical term for the rare phenomenon that sees twins fuse together in the womb and one develops physically inside another. 

This is a rare phenomenon and only 200 cases have been reported of which just 18 occurred inside the skull.

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Fetus-in-fetu has reportedly been detected in the pelvis, mouth, intestines, and even the scrotum. 

This happens when there is an 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 one of the twins grows, the smaller one gets absorbed into the abdomen and scientists suggest that this happens because 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.

This was published in December in the American Academy of Neurology's journal Neurology.

It is reported that the young girl was taken to the hospital after showing problems with her motor skills.

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It is added that the girl might have had 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 is not yet revealed by the doctors 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.

The siblings were connected to the host sibling via a single artery and vein, which the doctors said had been the umbilical cord.

Another case was reported in China back in 2015 when doctors found an unborn fetus inside the scrotal sac of its male twin.

The 20-day-old infant was taken to the 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.


Fetus-in-fetu (FIF) is a medical phenomenon that happens when a malformed fetus is enclosed within the body of its twin.

It reportedly occurs one in every 500,000 births and affects both males and females.

Fewer than 200 cases of the extremely rare condition have been reported in the 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.

Recovery can be made when the doctors remove the fetus through surgery.