Video report by ITV News Royal Editor Chris Ship
The Duke and Duchess of Cambridge are "very pleased" to announce they are expecting their third child, Kensington Palace has said.
The palace made the announcement as it confirmed the duchess would miss a public engagement on Monday because of severe morning sickness.
She suffered from the sickness, known medically as Hyperemesis Gravidarum, during her pregnancies with Prince George in 2013 and Princess Charlotte in 2015.
What have the royals said about the news?
Kensington Palace's statement said the Queen and members of both families were "delighted with the news".
Prince Harry was first to share his thoughts in public, hailing the news as "fantastic" as he arrived to meet health workers who were helping the victims of the Manchester terror attack.
The uncle said he was "very, very happy" for his brother and sister-in-law.
Her Majesty and the Duke of Edinburgh were performing official duties as the news was announced of their future great grandchild.
The couple were opening Scotland's Queensferry Crossing.
Clarence House also shared the Prince of Wales and the Duchess of Cornwall's joy at the news of another grandchild with a family photograph.
How will the royal line of succession be affected?
When born William and Kate's third baby will be fifth in line to the throne, a position currently occupied by Prince Harry.
The line of succession will be Prince Charles, Prince William, Prince George and Princess Charlotte, whose place is protected by the change in law in 2013.
The Succession to the Crown Act ended the more than 300-year rule that gave precedence to princes over their older sisters.
Had the duke and duchess always planned for a third?
A few statements made in public over the past few years have given an inkling to William and Kate's private plans for their expanding royal family.
The couple talked openly of having a family when they announced their engagement ahead of their wedding in April 2011.
The following year the prince, who has only Harry as a sibling, told teenagers on an overseas tour to Singapore he was "thinking about having two" children.
His wish was fulfilled with the birth of George in 2013 and Charlotte in 2015.
But the prospect of a third came up when Kate received a cuddly toy for newborns during a royal tour of Poland in July 2017.
She turned to the Duke and joked: "We will just have to have more babies."
The announcement comes in a significant week in the young family's life as Prince George begins school.
The four-year-old will attend Thomas's Battersea, an independent school in south London, with his first day on Thursday.
Kensington Palace's full statement: