Jessica Hay went public with the baby news last week (28Nov12) and now she's telling Life & Style magazine the royal tot will be a summer child.
And she has even opened up about Catherine and Prince William's bid to become parents, explaining, ""They've been preparing all summer. It was decided that they'd try at the tail-end of the royal tour in September, and that's when it happened!""
Hay attended the exclusive Marlborough school with Kate when they were both teenagers and she was a guest at the couple's wedding reception in 2011.
She reveals the mum-to-be is ""seven to eight weeks along"", and despite reports to the contrary following the Duchess' recent hospitalisation, Hay insists pregnancy agrees with her friend: ""Her face is already fuller. She really is glowing.""
And the school pal insists the date of the baby's birth will be extra special - because William's late mother Diana was born on 1 July and she wed his father Prince Charles on 29 July (12).