Zara, 36, and her rugby star husband Mike Tindall, are set to welcome their baby, a brother or sister for daughter Mia, in the summer.

The family spent Christmas in Sydney and flew to Queensland for the Magic Millions polo match which is part of a week-long festival also featuring racing and thoroughbred sales.

The Queen’s granddaughter is patron of Magic Millions Racing Women which promotes racehorse ownership among women.

The event is a favourite with the couple and champion horsewoman Zara has competed in the polo in previous years. 

Last year she took a tumble from her horse before getting back on and carrying on with the game leading Mike to announce through the loudspeaker: “That’s her will to win.

"She’s so tough, she bounces!” 

But this year she decided to take it easy and cheer on from the pavilion with Mike by her side.

Just over a year ago she suffered the devastating loss of a late miscarriage and Mike, 39, has explained how Mia, who will be four later this month, helped them through their grief. 

He said: “However we feel, she will come running up in our faces."

Zara’s baby will be 19th in line to the throne and the Queen is said to be “very pleased” at the news. 

The Royal Family is looking forward to a joyful 2018 with the birth of the Duchess of Cambridge third child expected in April and the wedding of Prince Harry and Meghan Markle in May.

Zara is expected to give birth in the same NHS hospital near her home in the Cotswolds in Gloucestershire, where Mia was born.