The final blow was delivered when Shelley became besotted with sixteen-year-old Mary Wollstonecraft Godwin. He fled abroad with her, and implored his wife to support this new relationship. He even invited her to join them in Switzerland. Harriet was distraught.

Only one of the Shelleys' four children survived. The first, a girl whose name, if she had one, has been lost to the ages, lived only a few weeks. William and Clara died as toddlers, and the Shelleys were childless before the birth of their fourth and last child, Percy Florence.

Pregnancy and childbirth, as well as death, was an integral part of Mary Shelley's young adult life. She had four children and a miscarriage that almost killed her.

Three-year-old William Shelley died of malaria on June 7, 1819, leaving Mary childless and emotionally exhausted. She suffered a mental breakdown, exasperated by Percy's affairs and lack of attention. However, her only child to survive his parents, Percy Florence, was born in November (Mellor 141–44).