Estela Melendez says doctors told her decades ago her stomach pains were due to an abdominal tumour.
Local media reported the 92-year-old grandmother recently suffered a fall, and went to a hospital in San Antoni, where doctors made the discovery.
X-ray tests revealed the presence of a foetus which weighed around 2 kilos.
Mrs Melendez said the large size was uncomfortable.
"I had no clothes to wear. I was very obese," she added.
The director of the hospital which discovered the foetus said it appears to have survived into the third trimester.
Her son, Luis, denies that his mother was ever informed of the pregnancy.
"The doctor never told her she was pregnant," he told a local journalist.
A calcified foetus is the result of a rare medical phenomena known as lithopedion where a foetus does not develop in the uterus but the abdomen.
Abdominal pregnancy reportedly occurs in every 11,000 pregnancies, of which only about 1.5 per cent develop into lithopedion.
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