Fetus of twin is surgically removed from the BRAIN of one-year-old girl in China

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 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.