Britain's Prince William and his wife Kate Middleton are expecting their third child.
Kensington Palace made the announcement on Twitter.
In a statement, the palace says: "The Queen and members of both families are delighted with the news."
The palace says as with her previous two pregnancies, she is suffering from Hyperemesis Gravidarum - very acute morning sickness.
It says this will impact on her public duties.
William and Kate already have two children: four year-old George and two-year-old Charlotte.
The statement says: "Their royal highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their third child.
"The Queen and members of both families are delighted with the news.
"As with her previous two pregnancies, the Duchess is suffering from Hyperemesis Gravidarum.
"Her royal highness will no longer carry out her planned engagement at the Hornsey Road Children's Centre in London today.
"The Duchess is being cared for at Kensington Palace. "
<blockquote class="twitter-tweet" data-lang="en"><p lang="en" dir="ltr">Read the press release in full ↓ <a href="https://t.co/vDTgGD2aGF">pic.twitter.com/vDTgGD2aGF</a></p>— Kensington Palace (@KensingtonRoyal) <a href="https://twitter.com/KensingtonRoyal/status/904633975381127168">September 4, 2017</a></blockquote>
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The latest addition to the Cambridge family will be a great-grandchild of Queen Elizabeth II and her husband, the Duke of Edinburgh.
He or she will be fifth in line to the British throne, pushing uncle Prince Harry into sixth place.
But it is unlikely that, as a third born royal in direct succession, he or she will become king or queen.
Additional reporting: IRN