Kate's Joy: Baby No.4!

The duchess and her prince are expanding their oh-so-cute family

Beaming in buttermilk yellow, Kate, Duchess of Cambridge was positively dazzling as she took to the balcony of Buckingham Palace for Trooping the Colour, along with her proud husband Prince William and their three adorable children.

Now, Woman’s Day can reveal just why the royal couple radiated such happiness and appeared so in love on the historic occasion: Kate, 37, is pregnant with a surprise baby – and the pair couldn’t be more delighted!

“William and Kate are expecting their fourth child,” a palace insider confirms. “Kate’s due in December, which will make the holidays even more exciting.”

‘HAPPY SURPRISE’

While a baby wasn’t on the agenda quite so soon – with Kate already juggling being a mum to Prince George, five, Princess Charlotte, four, and 13-monthold Prince Louis with her commitment to stepping up as Queen when the time comes – the insider says the news has come as a “happy surprise”.

While the Queen, 93, was the first to know, declaring she was “so pleased” with the news, other members of the royal family were told during the Trooping the Colour celebrations.

“Her Majesty holds William and Kate in such high regard. She adores her grandkids and greatgrandkids and looks forward to the new arrival,” the source tells.