Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge attend at a Creativity is GREAT reception at NeueHouse on Dec. 9, 2014 in New York City.

Catherine, Duchess of Cambridge, attends a Creativity is GREAT reception at NeueHouse on Dec. 9, 2014 in New York City.

Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge arrive at Metropolitan Museum of Art to attend the St. Andrews 600th Anniversary Dinner, Dec. 9, 2014, in New York City.

Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge attend the St. Andrews 600th Anniversary Dinner at Metropolitan Museum of Art, Dec. 9, 2014 in New York.

Catherine, Duchess of Cambridge attends the St. Andrews 600th Anniversary Dinner at Metropolitan Museum of Art, Dec. 9, 2014 in New York.

Prince William, Duke of Cambridge looks out from the observation deck of the Empire State Building with New York City Mayor Bill de Blasio, Dec. 9, 2014 in New York.

Prince William, Duke of Cambridge visits the observation deck of the Empire State Building, Dec. 9, 2014 in New York.

Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge watch an art performance as they visit The Door/City Kids Foundation, Dec. 9, 2014 in New York.

Prince William, Duke of Cambridge, and Catherine, the Duchess of Cambridge, leave the organization the Door after a visit, Dec. 9, 2014 in New York. The Door provides services to disadvantaged young people.

Britain's Prince William the Duke of Cambridge, right, and Kate, the Duchess of Cambridge attend a show presented by the organization The Door and the City Kids Foundation, Dec. 9, 2014 in New York.

Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge, pause for a moment of silence after laying a wreath at the National September 11 Memorial and Museum on Dec. 9, 2014 in New York City.

Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge walk through the National September 11 Memorial & Museum on Dec. 9, 2014 in New York City.

Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge visit the National September 11 Memorial & Museum on Dec. 9, 2014 in New York City.

Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge tour the lobby of One World Trade Center after a visit to the National September 11 Memorial & Museum on Dec. 9, 2014 in New York City.

Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge pose with basketball player LeBron James backstage as they attend the Cleveland Cavaliers vs. Brooklyn Nets game at Barclays Center, Dec. 8, 2014, in the Brooklyn borough of New York.

Britain's Prince William, third from right, and Kate, Duchess of Cambridge, during the second half of an NBA basketball game between the Brooklyn Nets and the Cleveland Cavaliers, Dec. 8, 2014, in New York.

Britain's Prince William and Kate, Duchess of Cambridge, watch the second half of an NBA basketball game between the Brooklyn Nets and the Cleveland Cavaliers, Dec. 8, 2014, in New York.

Britain's Prince William, left, talks with Jay-Z during the second half of an NBA basketball game between the Brooklyn Nets and the Cleveland Cavaliers, Dec. 8, 2014, in New York.

Catherine, Duchess of Cambridge, right, meets former United States Secretary of State, Hillary Clinton as Prince William, Duke of Cambridge meets Chelsea Clinton during a conservation reception at British Consul General's Residence on Dec. 8, 2014 in New York.

Catherine, Duchess of Cambridge chats with members of the British community in New York from culture, arts, hospitality, and business before a luncheon at the residence of the British Consul General, Dec. 8, 2014 in New York.

Catherine, Duchess of Cambridge, helps to wrap presents for children at the Northside Center for Childhood Development, Dec. 8, 2014 in New York.

Catherine, Duchess of Cambridge wraps presents for children with Chirlane McCray, first lady of New York City, and other volunteers at the Northside Center for Childhood Development, Dec. 8, 2014 in New York.

Britain's Prince William sits with World Bank President Jim Yong Kim before speaking on illegal transportation of wildlife parts across borders during an International Corruption Hunters Alliance event, Dec. 8, 2014, at the World Bank in Washington.

Britain's Prince William attends a a luncheon after speaking on illegal transportation of wildlife parts across borders during an International Corruption Hunters Alliance event, Dec. 8, 2014, at the World Bank in Washington.

Catherine, Duchess of Cambridge wraps presents for children with volunteer Mary Dawkins at the Northside Center for Childhood Development,Dec. 8, 2014 in New York.

Catherine, Duchess of Cambridge attends a pre-school class at the Northside Center for Childhood Development, Dec. 8, 2014 in New York.

Catherine, Duchess of Cambridge, accept flowers from a student outside Northside Center for Child Development, Monday, Dec. 8, 2014, in New York.

Catherine, Duchess of Cambridge greets guests as she visits Northside Center for Child Development during her official two-day visit to the United States, Dec. 8, 2014 in New York.

New York City first lady Chirlane McCray, left, and Britain's Catherine, Duchess of Cambridge, shake hands outside the Northside Center for Child Development, Dec. 8, 2014, in New York.

President Barack Obama meets with Prince William, Duke of Cambridge, in the Oval Office of the White House, Dec. 8, 2014 in Washington.

Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge leave a Gramercy Park residence, Dec. 7, 2014 in New York.

Catherine, Duchess of Cambridge and Britain's Prince William arrive at The Carlyle Hotel, Dec. 7, 2014, in New York.

Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge arrive to The Carlyle Hotel, Dec. 7, 2014 in New York.