“Baby born pregnant” is not a term you read every day, but the sensational headline isn’t just hyperbole; an infant girl was born in Hong Kong in 2010 with what doctors thought was abdominal tumors. Upon examination, it turned out she was carrying twin fetuses which had been absorbed while she was in the womb.

The fetuses were developed enough to have a spine, rib cage, four limbs, and anuses and were thought to be between 8 and 10 weeks old. Of course, they were conceived by the baby girl’s parents, but exactly how they ended up inside her is still a mystery to scientists, who say the condition is extremely rare and affects only one in 500,000 births around the world. Fewer than 200 cases have ever been reported.

The baby girl had a successful surgery to remove the fetuses, but the story made it into the Hong Kong Medical Journal as it is a subject of high interest to those in the medical field.

From the journal:

Despite a difference in the weight of the twin fetuses-in-fetu, the level of organogenesis was identical and corresponded to fetuses of 10 weeks of gestation. Each mass had four limbs, intact skin, rib cage, intestines, anus, ambiguous genitalia, primitive brain tissue and a spine with ganglion cells in the cord. Although considered a mature teratoma in the current World Health Organization classification, the theory of formation from multiple pregnancies has been commonly implied in more recent literature. The true aetiology of this rare condition remains unclear.

In other strange medical occurrences, a little over a year ago,an 82-year old Columbian woman went to the hospital complaining of stomach pains, only to discover that she had a nearly 40-year old fetus inside her. Sometimes called a “stone baby“, the phenomenon occurs when a fetus grows outside the uterus and the mother’s body begins to calcify it to protect herself from infection.