I forgot to ask if George was a boy or girl, Duke confesses to President

William joked about omitting to question medical staff about the one thing the world wanted to know as he chatted with the American leader in the White House’s famous Oval Office.

He added that he expects 2015 to be a “busy year”, with wife Kate due to give birth to the couple’s second child.

After the White House visit, William addressed a Washington conference to urge nations to redouble their efforts to combat the trade in illegal animal parts.

He described it as one of the world’s “most insidious forms of corruption”.

The call to action was part of his most important speech yet, in which he also told delegates that wildlife crime threatens the security of nations.

He said that in China, a huge market for endangered animal parts, the wholesale price of ivory has risen from $5 (£3) to $2,100 (£1,346) per kilo in 25 years and poaching has increased as a result.

The Duchess of Cambridge, meanwhile, was transformed into Princess Elsa from the hit movie Frozen – in the eyes of a group of New York children. The youngsters, who receive support from a Harlem-based mental health centre, welcomed Kate to their institution with the cry “Princess”.

Frozen tells the story of Anna’s adventures to find her sister Elsa whose icy powers have condemned the kingdom to winter.

Rose Ann Harris, director of early childhood services at the Northside Centre for Child Development, said about the children: “They thought she was the Princess from Frozen. When they heard she was coming they thought it was Elsa – we said a Princess and they got a Princess.”