Kensington Palace revealed the royal couple's happy news on Monday in England—but also confirmed that Middleton, 35, is once again suffering from the severe morning sickness that plagued her other pregnancies.
Baby number three will join Middleton and William's other two children, Prince George, 4, and Princess Charlotte, 2.
"Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their third child," a royal statement, released Monday morning British time, said.
"The Queen and members of both families are delighted with the news."
However, the statement also said that she has acute morning sickness, known as Hyperemesis Gravidarum, again.
As a result, Middleton, aka The Duchess of Cambridge, canceled a planned appearance Monday morning.
"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 statement read.
"The Duchess is being cared for at Kensington Palace."
Sources believe Middleton is less than three months pregnant and the palace was forced to make the announcement early because of her illness.
As Radar has previously reported, Middleton's morning sickness is serious.
"Hyperemesis gravidarum (HG) is a rare disorder characterized by severe and persistent nausea and vomiting during pregnancy that may necessitate hospitalization," according to WebMd.com.
"As a result of frequent nausea and vomiting, affected women experience dehydration, vitamin and mineral deficit, and the loss of greater than five percent of their original body weight."
When she was pregnant with Princess Charlotte, as PEOPLE reported, Kate was cared for at home at Apartment 1A of the palace.
Middleton had to cancel royal appearances during her previous bouts with morning sickness.
William's wife's appearance got alarming in 2014, when she appeared pin thin while pregnant.
Things got so bad with Middleton's health back then that she even fled to her parents' house during that time!
The pretty royal has marked the 20th anniversary of the death of William's tragic mother, Princess Diana, with her husband this summer.
And as Radar has noted, Middleton and William, 34, have had marriage counseling after his flirty ski vacation with party pals and pretty women in March.
But now, could a third child save the royal marriage? Stay tuned.
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