When asked by one American tourist called Mimi Ricketts about the impending arrival, Camilla instantly smiled.
"I said 'Are you excited about the new baby about to arrive in your family?' and she said yes she was."
Camilla, who was previously married to Andrew Parker Bowles, has eight grandchildren and step-grandchildren, with another on the way.
Her daughter Laura is mum to Eliza, and twins Gus and Louis, and her son Tom has two children, Lola and Freddy.
Meanwhile, stepson Prince William has three, George, Charlotte and Louis and Prince Harry is about to welcome his first child any day now.
"My own grandchildren call me GaGa," Camilla previously told the Daily mail.
"I don’t know if it’s because they think I am! It is funny but is still very sweet."