A one-year-old with an enormous head underwent surgery to have her identical twin’s foetus removed from her skull.

The unborn twin had continued to develop upper limbs, bones, and fingernails for months while within its sibling’s womb, according to doctors at Huashan Hospital at Fudan University in China. This is because of a condition called foetus-in-foetus.

The fetus was only discovered when the parents of the surviving twin took their daughter to the hospital due to her enormous skull and motor skills. It had been missed for a year after the girl’s birth.

A CT scan of the child, whose name has not been released, revealed that her unborn twin was pressing against her brain.

brain.

The foetus had managed to survive for a year inside the skull because it shares a blood supply with its sibling, but ultimately was surgically removed.

She also had hydrocephalus, a condition that causes a build-up of fluid in the brain that can cause an enlarged head, extreme fatigue and seizures.

Medical staff are unsure whether the surviving child will suffer long-term damage as a result of the strange condition.

Dr Zongze Li, a neurologist who treated the girl, said: “The intracranial foetus-in-foetu is proposed to arise from unseparated blastocysts.

“The conjoined parts develop into the forebrain of the host foetus and envelop the other embryo during neural plate folding.”

Foetus-in-foetu is an extremely rare phenomenon and has only been recorded 200 times, with 18 of these occurring in the brain.

The bizarre syndrome has also been seen in the pelvis, mouth, intestines and scrotum.

The unborn twin can survive and continue to grow for months inside its sibling, even growing organs and limbs.

The condition occurs when identical twins, who are formed when one egg splits, fail to separate completely in the womb, but doctors don’t know why this happens.

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