An incomplete fetus was discovered in the skull of a one-year-old girl!

A one-year-old girl in China was born with a fetus trapped inside her skull, doctors have revealed in a new case report.

Neurosurgeons operated on the baby, who had severe head swelling and developmental delays. But she died within two weeks because the damage to her brain was too severe for her to survive.

This extremely rare condition, known scientifically as fetus in fetu, affects about 1 in 500,000 births. Only 18 cases of it occurring inside the skull have been reported.

Doctors don't yet understand what causes this. But they do know that it happens during development in the womb, when identical twins, which are formed when two eggs split, don't separate.

After that, one twin gets stuck inside the other, and may continue to develop features, such as nails, hair, and limbs.

In 80% of cases, the absorbed fetal tissue settles in the abdomen, where doctors have a good chance of removing it without harming the patient. Other times, it is discovered in the baby's mouth, scrotum, or coccyx.

For example, in 2015, Chinese doctors successfully removed a fetus found in the scrotum of a 20-day-old infant.

But the condition is nearly 100 percent fatal when it occurs in the head, study authors Xiuyue Chen and Shuanling Chen, anesthesiologists from Peking University International Hospital in Beijing, China, wrote in the American Journal of Case Reports.

The case report notes that during a normal scan at 33 weeks, doctors discovered some "deformities" in the developing fetus's skull.

But the baby's birth was relatively natural, with doctors delivering her by caesarean section at 37 weeks gestation. Her head was larger than average, but she went home with her mother from the hospital.

A year later, she was admitted to Peking University International Hospital because her head was swollen and not growing normally.

The girl suffered from urinary incontinence, problems standing, holding her head up, and speaking anything except “Mom.”

So her doctors scanned her head, revealing a 5-inch mass in her skull, slightly larger than a baseball. Embedded in the mass were long pieces of bone.

At this point, doctors decided to perform surgery to try to remove the mass. They performed a craniotomy and removed part of her skull.

Inside, they found a white capsule containing a thick brown fluid and an immature fetus.

The fetus had a spine and bones, the beginnings of a mouth, eyes, hair, forearms, hands and feet. It was 18 cm long.

This caused "severe pressure on brain tissue." The patient never woke up after the operation and remained on life support.

Twelve days after surgery, the family decided to remove her from life support.

The doctors noted in the case report that the cause of these abnormalities "remains a mystery," but could be linked to environmental pollution, genetics, low temperatures, exposure to pesticides during pregnancy or problems with egg division.