The Colombian grandmother was thought to have gastroenteritis but an X-ray showed a petrified foetus in her abdomen.
Further tests at the emergency centre in capital Bogota revealed an 18-inch-long foetus, weighing 4lb, which is thought to have been inside the woman for at least four decades.
It is thought that the condition was caused by a rare medical phenomenon, called lithopedion.
This occurs when, unknown to the mother, a baby develops outside of the uterus then dies.
Unable to expel the foetus, the body covers it with calcium to protect the mother from infection, effectively turning it to stone.
The baby developed in the abdomen, and because the body generated defence mechanisms it was calcified
Dr Kemer Ramirez, from Bogata’s Tunjuelito Hospital, told Colombia’s El Tiempo newspaper: “This happens because the foetus moved from the womb to another place.
“In this case, the baby developed in the abdomen and because the body generated defence mechanisms it was calcified, staying there encapsulated.”
Doctors advised relatives that it was as risky to leave the foetus where it was as it was to remove it because of the woman’s advanced age.
The pensioner, who also suffers from Alzheimer’s, was later transferred to another hospital at the wishes of her family and will undergo surgery to remove the baby later this week.