The Duke and Duchess of Cambridge are expecting their third child, Kensington Palace has announced.
The Queen and both families are said to be “delighted with the news”.
As with her previous two pregnancies, the duchess, 35, is suffering from hyperemesis gravidarum, or severe morning sickness.
She pulled out of an engagement at the Hornsey Road Children’s Centre in London, which had been planned for Monday afternoon.
Catherine is being cared for at Kensington Palace, the statement said.
The duke and duchess have a son, George, who is four, and a daughter, Charlotte, aged two.
With the previous two pregnancies, the couple announced them before the 12-week mark – when most women have their first scan – because of the duchess being unwell with hyperemesis gravidarum.
Her first pregnancy was revealed when she was just a few weeks pregnant with Prince George after she was admitted to hospital in December 2012.
The duchess’s second pregnancy with Princess Charlotte was announced in September 2014, when she was treated at the palace for the condition.
Hyperemesis gravidarum affects about one in every 200 pregnancies and results in severe nausea and vomiting – with one of the main dangers being dehydration.
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.”
“Presumably his mother would be keen to take him to that, [but] whether she is going to be well enough to do that remains to be seen,” he told BBC News.
“It had also been expected that the Duke and Duchess of Cambridge would be taking a foreign trip this autumn,” he added.
“Whether they will be able to do that or whether the duchess will be well enough to do that also remains to be seen.”
The expected child will become the fifth in line to the throne behind Prince Charles, Prince William, Prince George and Princess Charlotte.
A change – which stops royal sons taking precedence over their female siblings in the line of succession – came into force in March 2015.
The child will be the Queen’s sixth great-grandchild.