The 23-year-old ambassador for World Contraception Day also revealed mum Sharon, who survived colon cancer, fears she will get the disease as she is on the Pill - or if she goes near a microwave.
"Because of her history she's convinced it causes cancer. I don't have a microwave in my house for the same reason," the Sun quoted her, as telling New! Mag.
Kelly also spoke about the moment when she first saw a condom aged seven while on tour with her dad's Black Sabbath band.
"Mum explained what they were. It was something my parents had never been shy about," she said.