What is hyperemesis gravidarum? Duchess Kate suffering from severe morning sickness with baby No. 3


Prince William and Princess Kate are expecting baby number three!

Kensington Palace announced the surprise news Monday morning in a statement.

Read: Duchess Kate Released From Hospital

"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 statement from Kensington Palace said. "The Queen and members of both families are delighted with the news."

RELATED: Duchess Kate's maternity style:

22 PHOTOS
Duchess Kate maternity style
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Duchess Kate maternity style
The Duchess wore her black pumps and carried a sparkly black clutch to complete her look.
The dress is a bit edgier than she typically chooses, but we loved it.
Just the tiniest hint of a bump! Kate is now 14 weeks pregnant.
The royal mom-to-be (round 2!) was glowing as she left the venue.
Kate attends the Action on Addiction Autum Gala in London on October 23, 2014, wearing a black Temperley London dress.
Kate, wearing a pale blue bespoke gown by Jenny Packham, arrives at the Natural History Museum on October 21, 2014.
The Duchess is the royal Patron of the Museum.
Kate dazzled on her first day back to royal duties since announcing her pregnancy.
The Duchess looked stunning in her floor length Jenny Packham gown.
She wore her hair down in bouncy curls with a delicate sparkly necklace.
Kate leaves the Natural History Museum.
The Duchess heads outside to present the awards of the evening.
Kate presents the Young Wildlife Photographer of the Year award to Carlos Perez Naval at the Natural History Museum.
Kate presents the Wildlife Photographer of the Year award to Michael Nichols at the Natural History Museum.
Kate leaves the Natural History Museum.
Prince William and Duchess Kate are seen as they welcome the President of Singapore Tony Tan Keng Yam at the Royal Garden Hotel on the morning of October 21, 2014. Kate is wearing a coat by Alexander McQueen and Jane Taylor hat. 
Kate's hair is pulled back in a low ponytail under her Jane Taylor hat. You can see the detail of the checked pattern of her Alexander McQueen coat.
The Duchess of Cambridge and Mary Tan depart the Royal Garden Hotel.
Kate smiles during the Horse Guards parade.
The duo prepare to meet the President of Singapore.
William and Kate laugh together during the Horse Guards Parade.
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The couple, who already have two children, 4-year-old George and 2-year-old Charlotte, reportedly announced the news prematurely because Kate, 35, suffers from severe morning sickness, also known as Hyperemesis gravidarum. She was forced to cancel a scheduled Monday morning appearance due to her condition.

"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 said.

Hyperemesis gravidarum affects about one in every 200 pregnancies and results in severe nausea, dehydration, and low blood pressure, among other things.

RELATED: Duchess Kate's style before becoming a royal:

18 PHOTOS
Duchess Kate's style before she became a royal
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Duchess Kate's style before she became a royal

Kate Middleton right before her graduation ceremony in 2005 rocking an oh-so-reliable white tank top (with a little bit of her tummy showing) and bootcut jeans.

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Howdy! Here's Kate Middleton at a music festival in 2005 looking very outdoorsy in a cowboy hat with some matching suede riding boots. 

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Kate Middleton definitely rocked cropped tops before it was even a trend! Here she is with her mom, Carole, in a simple white crop top that she paired underneath a blue blazer.

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Here's a full-length photo of her casual -- yet -- stylish look.

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Kate Middleton braved the frigid temps in 2006 with a cozy outfit of a Russian-style fur hat, a wool coat and a pair of her go-to suede boots.

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CHELTENHAM, ENGLAND - MARCH 17: Prince William's girlfriend, Kate Middleton wears a Russian style fur hat to the final day of Cheltenham Races on March 17, 2006 in Cheltenham, England. (Photo by Tim Graham/Getty Images)

Polka dot beauty! Kate Middleton cheered her future husband Prince William on at a polo match in 2006 in a bright red wrap dress.

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Flower power! Kate Middleton attended a 2006 party in a floral frock that hit just above her knees. She paired the look with a light pink crushed velvet jacket and some seriously pointy flats.

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Kate Middleton's off-duty style was impeccable. At another polo match in 2006, she rocked white flared jeans and a tummy-bearing sheer top.

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A close-up look at Kate's super casual ensemble.

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Kate Middleton looked fabulous in a black dress with tights and suede knee-high boots while out on the town in 2007.

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Kate Middleton made denim skirts a thing before they even became trendy. Here she is on a casual shopping trip in 2007.

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What a party animal! Before becoming a royal, Kate Middleton was just a regular 20-something who went out at the hottest clubs in London. Of course, she always looked great for the occasion. 

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Before Kate Middleton was wearing tiaras and fancy pearls, she was the queen of accessorizing. Between the scarf and the French-inspired beret, she looked absolutely flawless in 2007.

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Another close up look of Kate's chic accessorizing.

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Uggs -- oh my! Even Kate Middleton couldn't resist the comfy boots that blew up in popularity in the early 2000s. Here she is in 2007.

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While Kate rocks multi-million dollar jewels now, she still loved to accessorize even before she became a royal. We're totally loving that chunky white necklace!

(Getty)

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Dr. Jennifer Ashton told Good Morning America that the condition can be dangerous.

"We're talking about the potential for severe malnutrition, electrolyte abnormalities, sodium and in some cases organ failure... with HG you're talking about vomiting that could be every hour or every other hour," she said.

Kate is being cared for at Kensington Palace. She is reportedly less than three months pregnant.

Read: Paralyzed Woman Announces Pregnancy During Half-Marathon: 'I Like to Challenge Myself'

With Kate's previous two pregnancies, the couple also announced before the three-month mark because of the Duchess' condition. She was hospitalized due to Hyperemesis gravidarum while she was a few weeks pregnant with Prince George in 2012.

The birth will bump Prince Harry down to sixth in line to the throne.

Watch: Parents Say Sonogram Shows Jesus' Face in Womb With Baby

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