An elderly woman in Colombia got the surprise of her life when doctors discovered a calcified fetus inside her abdomen. The “stone baby,” known in the medical community as a lithopedion, weighed four pounds and was 40 years old.

According to CBS News, the unidentified woman, who was 82, went to the hospital on Dec. 8 complaining of stomach pains. Physicians initially thought the woman suffered from gastroenteritis, an infectious form of diarrhea. It was only after hospital staff performed a radiography exam that they discovered the calcified fetus, or stone baby, inside her stomach.

"In the medical exam the medic who was looking over [the patient] noted something abnormal in her abdomen and ordered an exam thinking that perhaps it was gallstones,” Dr. Kemer Ramirez from Bogota’s Tunjuelito Hospital, where the woman with the stone baby was admitted, told local media, according to NTD Television. “An ultrasound was done and it wasn't positive. Then, an abdominal radiography was ordered which located a tumor in the abdominal cavity which concluded that it was a fetus in the woman's abdomen.”

Local media reported that the stone baby was 40 years old. The woman will be treated at another hospital to have the calcified fetus removed.

Lithopedion is a very rare phenomenon. Since the late 16th-century, there are only about 300 cases of stone babies accounted for in medical journalism. According to New York Daily News, the first documented case of a calcified fetus occurred in France in 1582 when doctors discovered a 28-year-old calcified fetus inside a 68-year-old woman.

Lithopedion occurs during an ectopic pregnancy when the fetus begins to grow outside the mother’s uterus and in the abdomen. This occurs in about one in every 11,000 pregnancies; lithopedion happens in roughly 1.5 to 1.8 percent of these, according to the Sao Paolo Medical Journal.

“Usually an ectopic pregnancy will mean a [fallopian] tubal pregnancy, but in a small percentage of cases, the pregnancy can actually occur in the abdominal cavity -- in places like the bowel, the ovary, or even on the aorta,” Dr Natalie Burger, an endocrinologist and fertility specialist in Texas, told NBC News in 2009. “These are very rare locations and they can be very dangerous.”

Calcifying the fetus is the body’s way of protecting the mother from being infected by the fetus’s dead tissue.

In the 300 or so cases of stone baby documented in medical literature, the patients’ ages ranged from 23 to 100 years old, but the majority of them were over 40. The calcified fetuses in those cases ranged in age from 4 to 60 years, and fetal death occurred between at full term in 43 percent of the cases, the Sao Paulo Medical Journal noted.

Oftentimes, women carrying a lithopedion baby won’t even know it’s there. Some women will even give birth to other children while the calcified fetus is still in her abdomen. If a woman does not have the money to surgically remove the stone baby, she may just ignore it.