In September 1983, roughly a year after the birth of William, Buckingham Palace announced that Diana was pregnant for a second time. However, within a week, Diana suffered a miscarriage when on holiday at Balmoral, Scotland.
In 1984, she became pregnant again and gave birth to Harry.
American tabloid speculation of Kate's pregnancy has been rampant for months. One newspaper even cited anonymous sources talking about Kate's hormone levels. Others have focused on the first signs of the royal bump.
The palace said the royal family was "delighted" by the news. 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 his confidence the duo will make "absolutely brilliant parents."
The leaders of Britain and the 15 former colonies that have the monarch as their head of state agreed in 2011 to new rules which give females equal status with males in the order of succession.
Although none of the nations had legislated the change as of September, the British Cabinet Office confirmed that this is now the de-facto rule.
Those changes make Kate's pregnancy all the more significant for the royal family, according to 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."
Graham Smith of anti-monarchy group Republic called Kate's pregnancy a "private, personal matter" for her and William, saying the flood of media coverage was disproportionate
"We've heard today that our future head of state is on the way. It's a pretty bizarre way of choosing someone for public office," he said.
On the couple's tour of Malaysia, Singapore, the Solomon Islands and Tuvalu in September, William reportedly said he hoped he and Kate would have two children.
Associated Press writers Jill Lawless, Paisley Dodds and AP Medical Writer Maria Cheng contributed to this report.
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