British Prime Minister David Cameron admitted he got a heads-up about the pregnancy, saying he found the news "quite difficult" to keep to himself and expressing confidence the young couple will make "absolutely brilliant parents."
The pregnancy comes after a 2011 decision by the leaders of Britain and the 15 Commonwealth nations endorsing new rules that give girls equal status with boys in the order of succession.
Those changes make Kate's pregnancy all the more significant for the royal family, said Ingrid Seward, editor-in-chief of Majesty magazine.
"This is the first child who will be an heir to the throne, whatever sex they are," she said. "It's a new beginning."