In the chilly night air at London's Camden market, concertgoers and shoppers seemed surprised by the news — though all agreed that it had been widely anticipated.
"It feels a lot like a Christmas present for the nation!" said Ravian Van Den Hil, a Dutch student studying in London. "It makes me feel quite happy."
Others wondered why Britain continues to spend so much to support the royal family. "I don't think it's a good thing," said Stephen Jowitt as he strolled down Camden High Street. "It reinforces a class system."
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 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."
Like Kate, William's mother, Diana, also reportedly suffered from morning sickness for months, and was the subject of constant media attention after she became pregnant just four months after her wedding to Prince Charles. "The whole world is watching my stomach," Diana once said.
According to Britain's Department of Health, severe morning sickness most often affects women early in their pregnancy, and is more common in young women, women who are pregnant for the first time and those expecting multiple babies.
Dr. Daghni Rajasingam, a spokeswoman for Britain's Royal College of Obstetricians and Gynaecologists, said women with severe symptoms — including dehydration, dizziness and persistent vomiting — need to be hospitalized for treatment, including being given fluids intravenously.
"However, this usually only means a few days in (the) hospital," she said in a statement. "The best advice for anyone suffering from (severe morning sickness) is to get plenty of rest and drink lots of fluid."
Associated Press writers Jill Lawless, Paisley Dodds, Danica Kirka and AP Medical Writer Maria Cheng contributed to this report.