"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 palace statement read. "The Queen and members of both families are delighted with the news."
As with her previous two pregnancies, Middleton is suffering from hyperemesis gravidarum, which is "very acute morning sickness, which may require supplementary hydration, medication and nutrients," according to the statement.
The illness forced Middleton, 35, to cancel an engagement Monday morning at the Hornsey Road Children's Center in London.
She is being cared for at Kensington Palace, the statement said.
CNN reported the duchess is not yet past the 12-week mark of her pregnancy.
After William and Kate wed, a change in law gave women the same rights of accession to the throne as men.
Queen Elizabeth, 91, has five other great-grandchildren.
Prince George will start school at Thomas' Battersea in south London on Thursday.
Before William's grandfather, Prince Philip, retired from public duties in August, the family moved back to the British capital fulltime so that they can take on more royal duties. Their country home has been Amner Hall for the past three years.
In London, their residence has been Apartment 1A at Kensington Palace since 2014.
Prince William, 35, is an air ambulance helicopter pilot.