Britain's Prince William and Duchess Catherine cried with happiness when they discovered they were expecting their first child.

The couple will welcome the third-in-line to the British throne next year and the Duke and Duchess of Cambridge - who tied the knot in April 2011 - are over the moon.

Catherine's friend Jessica Hay said: ''William and Catherine are absolutely overjoyed with becoming parents. It's the most thrilling news to end a wonderful year on.''

While the couple only announced their pregnancy this week, Jessica says it wasn't a shock to their friends as they had been preparing to conceive for some time.

She added to America's Life & Style magazine: ''Catherine's been steadily putting on weight since April this year and hasn't been drinking for a good few months - 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 what happened.

''The difference in Catherine is already tremendous. Her face is already fuller, she really is glowing.''