The world has had its first glimpse of Prince William and Kate's new baby girl, with the trio posing for photographers before leaving St Mary's hospital in London.The Duke and Duchess of Cambridge left the hospital with their new baby girl on Saturday less than 10 hours after Kate gave birth at 8.34am local time.US President Barack Obama and hiswife Michelle spoke of their joy after Prince William's wife Kate gave birth to a baby girl today."Michelle and I are delighted to congratulate the Duke and Duchess of Cambridge, her Majesty the Queen and the royal family, and all the people of the United Kingdom on the birth of the Royal Princess," the White House said in a statement.