Days later, the Duke of Cambridge opened up about the news while speaking at a press conference on mental health in Oxford.
The BBC's royal correspondent Nicholas Witchell said the announcement comes at the start of a "significant week" for the family "because Prince George is due to start at big school". He also noted that "There's not much sleep going on at the moment".
He continued to say that: "It was a bit anxious to start with" but that they can "start celebrating this week" - something which might hint that Kate is approaching the 12-week mark. But unfortunately, Kate Middleton is suffering from the same condition she had during her first two pregnancies: hyperemesis gravidarum (HG). Her mother, Carole Middleton, was also found near the royal residence on Monday, 4 September.
Prince Harry described the news of his sister-in-law's pregnancy as "fantastic". "They are favoring very traditional names, such as Phillip, or Andrew for a boy, and Elizabeth or Marguerite for a girl".
On how the Duchess was, he replied: "I haven't seen her for a while but I think she's okay".
Although the due date has not been announced yet, there are a couple of educated guesses.
It concluded: "The Duchess is being cared for at Kensington Palace". The current royal succession is: Charles, William, George, Charlotte, then Harry.
"But she's ecstatic to be expecting baby number three - this is everything she's always wished for".
The Duchess was last seen in public when she, Prince William and Prince Harry visited a garden paying tribute to Princess Diana on the 20th anniversary of her death.