LONDON, ENGLAND - MARCH 17: Prince William, Duke Of Cambridge and Catherine, Duchess of Cambridge talk with soldiers of the 1st Battalion Irish Guards following their St Patricks day parade at Cavalry Barracks on March 17, 2017 in London, England. (Photo by Richard Pohle - WPA Pool/Getty Images)

LONDON, ENGLAND - MARCH 17:  Prince William, Duke Of Cambridge and Catherine, Duchess of Cambridge talk with soldiers of the 1st Battalion Irish Guards following their St Patricks day parade at Cavalry Barracks on March 17, 2017 in London, England. (Photo by Richard Pohle - WPA Pool/Getty Images)

LONDON, ENGLAND - MARCH 17: Prince William, Duke Of Cambridge and Catherine, Duchess of Cambridge take a drink of Guinness as they meet with soldiers of the 1st battalion Irish Guards in their canteen following their St Patricks day parade at Cavalry Barracks on March 17, 2017 in London, England. (Photo by Richard Pohle - WPA Pool/Getty Images)

LONDON, ENGLAND - MARCH 17:  Prince William, Duke Of Cambridge and Catherine, Duchess of Cambridge take a drink of Guinness as they meet with soldiers of the 1st battalion Irish Guards in their canteen following their St Patricks day parade at Cavalry Barracks on March 17, 2017 in London, England. (Photo by Richard Pohle - WPA Pool/Getty Images)

LONDON, ENGLAND - MARCH 17: Catherine, Duchess of Cambridge and Prince William, Duke Of Cambridge sit for an official photograph with warrant officers of the 1st Battalion Irish Guards following their St Patricks day parade at Cavalry Barracks on March 17, 2017 in London, England. (Photo by Richard Pohle - WPA Pool/Getty Images)

LONDON, ENGLAND - MARCH 17:  Catherine, Duchess of Cambridge and Prince William, Duke Of Cambridge sit for an official photograph with warrant officers of the 1st Battalion Irish Guards following their St Patricks day parade at Cavalry Barracks on March 17, 2017 in London, England. (Photo by Richard Pohle - WPA Pool/Getty Images)

Britain's Prince William, Duke of Cambridge, (L) and his wife Britain's Catherine, Duchess of Cambridge react whilst talking with off-duty soldiers from the 1st Battalion Irish Guards, after watching their annual St Patrick's Day parade at Cavalry Barracks in Hounslow, west London on March 17, 2017. / AFP PHOTO / POOL / RICHARD POHLE (Photo credit should read RICHARD POHLE/AFP/Getty Images)

Britain's Prince William, Duke of Cambridge, (L) and his wife Britain's Catherine, Duchess of Cambridge react whilst talking with off-duty soldiers from the 1st Battalion Irish Guards, after watching their annual St Patrick's Day parade at Cavalry Barracks in Hounslow, west London on March 17, 2017. / AFP PHOTO / POOL / RICHARD POHLE        (Photo credit should read RICHARD POHLE/AFP/Getty Images)

Britain's Prince William, Duke of Cambridge, (L) and his wife Britain's Catherine, Duchess of Cambridge react as they each hold a pint of Guinness whilst talking with off-duty soldiers from the 1st Battalion Irish Guards, after watching their annual St Patrick's Day parade at Cavalry Barracks in Hounslow, west London on March 17, 2017. / AFP PHOTO / POOL / RICHARD POHLE (Photo credit should read RICHARD POHLE/AFP/Getty Images)

Britain's Prince William, Duke of Cambridge, (L) and his wife Britain's Catherine, Duchess of Cambridge react as they each hold a pint of Guinness whilst talking with off-duty soldiers from the 1st Battalion Irish Guards, after watching their annual St Patrick's Day parade at Cavalry Barracks in Hounslow, west London on March 17, 2017. / AFP PHOTO / POOL / RICHARD POHLE        (Photo credit should read RICHARD POHLE/AFP/Getty Images)