William was on the opposite team to his brother, Prince Harry, at the event held at musical impresario Andrew Lloyd Webber's Berkshire estate.
The Kent and Curwen Royal Charity Polo Cup was held in aid of Sentebale and the Tusk Trust charities, which the Princes support.
Harry's team won the game with a last-minute goal, and he was awarded the most valuable player award.
Meanwhile bookmakers Coral said the odds on Kate giving birth tomorrow had shortened from 5/1 to 4/1 after a flurry of bets.
Duchess of Cambridge portrait unveiled
The Duchess of Cambridge after viewing a new portrait by Paul Emsley of herself at the National Portrait Gallery in central London.
Royal visit to Aldershot
The Duchess of Cambridge visit the 1st Battalion Irish Guards to attend the St. Patrick's Day Parade at Mons Barracks, Aldershot.
Royal visit to CBUK
The Duchess of Cambridge as she and her husband, the Duke of Cambridge, visited the headquarters of the charity CBUK, Child Bereavement UK at Saunderton Buckinghamshire.
Horse Racing - 2013 Cheltenham Festival - Day Four - Cheltenham Racecourse
Catherine, Duchess of Cambridge attends the Cheltenham Festival on Gold Cup Day on March 15, 2013.
Duchess of Cambridge
Britain's Kate, Duchess of Cambridge, right, accompanies Queen Elizabeth II and Prince Philip, obscured at left, as they arrive at Baker Street underground station in London for a visit to mark the 150th anniversary of the London Underground, Wednesday, March 20, 2013. (AP Photo/Sang Tan)
Britain's Duchess of Cambridge, Chief Scout Bear Grylls and Sea Scout Rob Butcher, right, attend the National Review of Queen's Scouts at Windsor Castle, near London Sunday, April 21, 2013. (AP Photo/Olivia Harris, Pool)
Kate visit to the National Portrait Gallery
The Duchess of Cambridge arriving for a visit the National Portrait Gallery, London, to celebrate the work of The Art Room charity.
Britain's Prince Charles, left, Camilla Duchess of Cornwall, centre, and Kate Duchess of Cambridge attend a Garden Party hosted by Queen Elizabeth II in the grounds of Buckingham Palace, central London Wednesday May 22, 2013. (AP Photo/ John Stillwell/PA) UNITED KINGDOM OUT NO SALES NO ARCHIVE
FILE - This is a Friday, April 26, 2013 file photo of Britain's Kate the Duchess of Cambridge arrives with her husband Prince William, not pictured and his brother Prince Harry, rear right, to attend the inauguration of "Warner Bros. Studios Leavesden" near Watford, approximately 18 miles north west of central London. s it a boy? A girl? As the due date for Prince William and the former Kate Middleton's first child nears, observers are also asking: Where will the royal baby spend its first few months? The world wont know the gender of the baby until it's born in July _ and the answer to the second question isn't as straightforward as one might think. (AP Photo/Chris Jackson, Pool, FILE)
Kate, the Duchess of Cambridge, left, arrives for an official visit to Naomi House near Winchester, England, Monday, April 29, 2013. Naomi House is a care home for children and young adults who have terminal conditions. (AP Photo/Alastair Grant)
Britain's Kate the Duchess of Cambridge arrives with her husband Prince William, not pictured and his brother Prince Harry, rear right, to attend the inauguration of "Warner Bros. Studios Leavesden" near Watford, approximately 18 miles north west of central London, Friday, April 26, 2013. As well as attending the inauguration Friday at the former World War II airfield site, the royals will undertake a tour of Warner Bros. "Studio Tour London - The Making of Harry Potter", where they will view props, costumes and models from the Harry Potter film series. (AP Photo/Chris Jackson, Pool)
Royals attend sports project launch
The Duke and Duchess of Cambridge and Prince Harry during a visit to Bacon's College in Greenwich, London to attend the launch of a new sports project that aims to train the next generation of sports coaches.
60th anniversary Queen Elizabeth II's coronation
The Duchess of Cambridge and Prince Harry as they attend a service at Westminster Abbey in central London to mark the 60th anniversary of the Queen's coronation.