Princess Kate and Prince William Expecting Baby No. 3!

And the Royal Fab 4 are soon to be five!

Princess Kate and Prince William are expecting their third child together, Kensington Palace confirmed today. The royals are already parents to 4-year-old Prince George and 2-year-old Princess Charlotte.

"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, released Monday morning, said.

"The Queen and members of both families are delighted with the news."

During her first two pregnancies, Kate, 35, battled acute morning sickness, also known as Hyperemesis Gravidarum. She is also afflicted with the condition during this pregnancy, according to the statement.

As a result, she 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 said.

"The Duchess is being cared for at Kensington Palace."

Kate is understood to be less than three months pregnant – the palace made the announcement early because of her illness. Last time, when she was pregnant with Princess Charlotte, Kate was also cared for at home at Apartment 1A of the palace.

The birth will relegate Prince Harry – who is visiting Manchester today – down to sixth in line to the throne.

During an appearance Monday, Harry told reporters the news is" "fantastic, great. Very, very happy for them." Asked about Kate he added, "I haven't seen her for a while but I think I she's okay."

The announcement comes in a landmark week for the family. On Thursday, she and Prince William are set to take Prince George to school for his first day at Thomas's Battersea.

The newest royal will be joining an adorable set of siblings. George and Charlotte have been charming people around the world since their respective births — including on the family's fall 2016 tour of Canada and on Christmas Day in Kate's hometown of Bucklebury.

The royal brother and sister had a blast this past September during a playdate on the Canadian tour that saw George discovering the joy of bubbles and Charlotte speaking her first words in public.

The arrival of Will and Kate's third child could be historic: If the baby is a boy, he will not displace big sister Charlotte in the line of succession, thanks to a measure that passed just after Will and Kate's 2011 wedding. Up until this change, a younger son would usurp his older sister's place when it comes to inheriting the throne.