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

Neurosurgeons performed surgery on the child, who was suffering from severe head swelling and developmental delays. But she passed away within a fortnight because the damage to her brain was too severe to survive.

This exceedingly rare condition, known scientifically as fetus in fetu, affects roughly 1 in every 500,000 births. Only 18 cases of this occurring within the skull have ever been reported.

Doctors don’t understand yet what causes it. But they know it occurs during development inside the womb – when identical twins, formed when two eggs split, don’t separate completely.

Then, one twin gets stuck inside the other, and may continue developing features like fingernails, hair and limbs.

In 80 percent of cases, the absorbed fetal tissue gets lodged in the abdomen, where doctors have a high chance of removing it without harming the patient. Other times, it’s been identified in a child’s mouth, scrotum or tailbone.

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

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

The report states that during a normal check up at 33 weeks, doctors discovered some ‘abnormalities’ in the developing embryo’s skull.

But her birth was fairly standard – doctors delivered her by caesarean section at 37 weeks. Her head was bigger than average, but she went home with her mother from the hospital.