Rosemary Letts was born in Northam, Devon, to William Andrew "Bill" Letts (25 February 1921 – 24 May 1979) and Daisy Gwendoline Fuller (born 1919) after a difficult pregnancy. She was the fifth of seven children born into a poor family. Rose's mother suffered from depression and was given electroconvulsive therapy (ECT) while pregnant; some have argued that this treatment may have caused prenatal injuries to her daughter. Daisy received her last session of ECT just days before Rose was born. Rose herself grew up into a moody and precocious teenager, prone to daydreaming and performing poorly at school.