The Royal Couple have four-year-old son George and two-year-old daughter Charlotte
Prince William and Kate expecting third child
Prince William and his wife Kate are expecting their third child, Kensington Palace announced on Monday, adding that Queen Elizabeth II was "delighted".
In a statement, the palace said that 35-year-old Kate was suffering from hyperemesis gravidarum, a severe form of morning sickness which also marked her previous pregnancies, and would not attend a planned event later Monday.
The Duke and Duchess of Cambridge already have two children – George, aged four, and Charlotte, aged 2.
Prince Harry welcomed the news, telling reporters he felt "fantastic" about becoming an uncle for the third time. He added that he was “very, very happy” for his brother and sister-in-law.
Prince Charles was also “delighted” with the news, Clarence House said.
The new baby will be fifth in line to the throne, after the Prince of Wales, William, George and Charlotte. Harry will drop into sixth place.
The Duchess recently dropped a hint that she was ready for more children, during a trip to Warsaw in July.
Upon receiving a cuddly toy designed for newborns at a reception in the Polish capital, she turned to her husband and joked: “We will just have to have more babies”.
Kate’s last public appearance was at a tour of Kensington Palace’s White Garden last week, at an event to commemorate the anniversary of the death of Diana, Princess of Wales.
Number 10 tweeted a message of congratulations from Prime Minister Theresa May, which said: "This is fantastic news. Many congratulations to the Duke and Duchess of Cambridge."
The announcement comes just days before Prince George starts school. The toddler is enrolled at an independent school in Battersea, south London, where fees are upwards of £17,000 per year.
Thomas’s Battersea will welcome the young prince on Thursday, when term starts.
The school is conveniently located just around the corner from Kensington Palace, where the Cambridges are now mainly based, having moved down from their Norfolk home Amner Hall.