Archaeologists discovered this ovarian teratoma, or a tumor that had started sprouting teeth, in a burial outside the Church and Convent of Carmo in Lisbon.

Archaeologists excavating a gothic church graveyard in Lisbon, Portugal, made a discovery for the annals of medical history: an ovarian tumor that had started forming teeth.

Today, doctors know that this type of cyst, called a teratoma, is the most common tumor that occurs in the ovaries. But scientists are just starting to learn about past teratoma cases thanks to new evidence from the archaeological record.

A teratoma, which essentially translates as "monstrous swelling" from Greek, can occur when cells that should become eggs start multiplying abnormally and form mature tissues like hair, teeth and bones. [25 Grisly Archaeological Discoveries]

These cysts account for up to 20 percent of all ovarian tumors, and most develop in women of reproductive age, according to past studies. These masses are usually benign and go unnoticed, without causing any symptoms. But some can be cancerous, and some can grow so large that they cause severe pain, or twisting in the ovaries. The largest reported teratoma was 18 inches by 10 inches (45 by 25 centimeters), removed from a 74-year-old woman, according to one review.

While many teratomas look like balls of tissue, some can develop so much that they take the shape of a fetus. In 2004, doctors in Japan reported the discovery of a "doll-like" teratoma with a head and limbs in the ovary of a 25-year-old virginal woman.

The tumor newly unearthed in Portugal measures 1.7 inches (4.3 cm) at its widest point, according to a study published May 12 in the International Journal of Paleopathology. The mass is embedded with at least five malformed teeth, and it shows signs of some disorganized bone formation.

Researchers discovered the tumor during the excavation of 42 burials outside the Church and Convent of Carmo in Lisbon in 2010 and 2011. The calcified mass was resting near the pelvic area of a woman who was over 45 years old at the time of her death, the study said. This cemetery was used from the early 15th century until the devastating 1755 earthquake that wrecked the church and many other buildings in Lisbon, so the researchers assume the woman lived sometime during that era, the study said.

"When the archaeologists found this ovarian mass, obviously they immediately noticed they were in the presence of a very unusual thing that should be carefully recovered and transported for further analysis in the laboratory," study leader Sofia Wasterlain of Portugal's University of Coimbra told Live Science. "However, at that time they didn't know what it was exactly."

Wasterlain and her colleagues considered other explanations for this little bony ball, such as a dead fetus or an ectopic pregnancy (where the embryo attaches outside the uterus) that calcified within the woman's body. But they concluded that this case looks most like a teratoma. It's not possible to tell if the tumor had any effect on the life or death of the woman, but her skeleton didn't seem to have any changes related to the tumor, the report said.

"Some types of tumors that are thought to be characteristic of modern societies and commonly attributed to Western civilization are also found in past populations," the researchers wrote in the study. "This case also draws attention to the importance of conducting meticulous archaeological excavation in order to preserve rare, but significant findings. During excavation of human remains, materials from body cavities, which may provide clues not directly accessible from the skeleton, should always be sought and recovered with care."

This case in Portugal is not the first time a teratoma like this has been unearthed in a graveyard. In 2013, archaeologists digging at a Roman necropolis in Spain reported that they found the 1,600-year-old remains of a woman who had a calcified tumor in her pelvis.

Original article on Live Science.