Her overjoyed musician husband added: "Sorry for my twitter silence over the last 24hrs, I’ve been far too busy cuddling my son… :)
"Yep, @MrsGiFletcher gave birth to our insanely gorgeous son yesterday at 7pm. He is healthy, beautiful and I’ve already changed his nappy."
Soon after the pair posted their happy news, "#BabyFletcher" started trending on Twitter as fans rushed to send their congratulations and retweet the announcement.
In her latest blog, Giovanna told how she had reached 37 weeks – classed as a full term pregnancy – and was looking forward to welcoming the new baby, who the couple nicknamed "Crumb".
"And now we've arrived at 37 weeks… from this point on Crumb can arrive whenever he/she chooses. It’s mindboggling to think that the little human I’ve been growing for the last eight and a half months is about to come out and meet us. I can't wait!"
Video: Giovanna shows off her bump as she sings Love Is On The Radio with Tom
In the week leading up to her birth Giovanna – who had just started her maternity leave – did a one-on-one session with her hypnobirthing teacher Hollie. The novelist was looking forward to using the relaxation techniques she had learned to help her during labour.
Speaking about her hypnobirth course experience on her blog, she wrote: "I was ready to try anything to help get me through the ordeal… what I wasn’t prepared for was just how lovely I’d find hypnobirthing, and how it would change my whole perception on the day we welcome our little Crumb into the world."