It’s very rare fewer than 300 cases have been noted in 400 years of medical literature.

In 1955 a 26 year old woman named Zahra Aboutalib was rushed to the hospital to deliver her baby, but after and excruciating 48 hour labor, she still could not deliver the baby so she had to undergo a caesarean section but the thought of dieing in child labor terrified the woman and urged her to flee the hospital and hide in her home. After doing so, she gave birth nearly 50 years later to a mummy baby.

There is a specific term for this called Lithopedion is better known as a stone baby. 

Source: Mind Blowing Facts