Stone baby, 18th-century illustration. Also known as a lithopedion, this occurs when a foetus is retained in a woman's uterus and becomes calcified. After the death of the foetus, calcium salts are deposited in the foetal sac and it is encased by a shell or even partly calcified itself. There are cases where such mummified foetuses have been carried for years in the uterus of the mother. This one has been in the uterus for 26 years. It is based on the French anatomy waxwork displays by Arneau. This illustration is from 'Osteographia, or the Anatomy of the bones' (London, 1733) by English surgeon and anatomist William Cheselden (1688-1752).