A calcified fetus -- not the one most recently found -- that remained in its mother for 50 years. (Otis Historical Archives of “National Museum of Health & Medicine”)

It's a rare condition that admittedly sounds like something out of a horror movie: A fetus dies, and is then calcified -- essentially turned to stone -- inside its mother's body.

In June, reports of a case in Chile circulated – one that may never have been discovered if not for an unrelated injury. Now Estela Meléndez, the 91-year-old resident of La Boca who went to the hospital after falling and learned shocking news, has spoken with CNN about her ordeal.

[Newborn girl might have been carrying her parasitic twins]

"The doctors said I had a tumor and that they needed to operate on me," Meléndez says in the video interview. But a second x-ray confirmed not a tumor, but a fetus.

This phenomenon is called a lithopedion, otherwise known as a stone baby. First described back in the 10th century, the calcification usually occurs when an abdominal pregnancy -- one that occurs outside the womb, somewhere inside the mother's abdomen -- goes awry. Nearly all abdominal pregnancies will end in miscarriage, but if the fetus is small enough, the mother's body can reabsorb the tissue without a problem. If the fetus is too big, surgical intervention is necessary.

And if that doesn't happen, a lithopedion can form.

[An adult woman was found to be missing her cerebellum]

Meléndez has had the small bump caused by her stone baby for over half a century now, and while it causes occasional pain, she hasn't had any serious health issues because of it. In fact, forming a stone baby is actually a brilliant protective measure on the part of a mother's immune system: The calcified layer that forms around the baby protects its mother from its dead tissue, which would otherwise likely cause an infection.

Some patients with abdominal lithopedions have been known to carry healthy pregnancies at the same time. Unfortunately for Meléndez, this 4.4-pound stone baby formed inside her uterus instead of in the abdomen, which prevented her from having living children.

Because of Meléndez's age, doctors have opted not to risk removing the fetus.

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