The Archbishop of Canterbury tweeted: "Wonderful news that the Duke & Duchess of Cambridge are expecting second child. Praying for the family, and that morning sickness subsides."
Kensington Palace confirmed Kate was suffering from Hyperemesis Gravidarum, a type of morning sickness which she had during her first pregnancy.
She'll no longer accompany The Duke of Cambridge to an event in Oxford and is now being treated by doctors at home.
The Queen and members of the Royal Family are said to be delighted with the news.
Shirley Tart is the Royal Correspondent for the Shropshire Star and a warden at her local church.
She thinks excitement over the second baby could top that of the first.
Speaking on the News Hour she said: "It depends a lot on the moment, when the baby's born, what the climate's like.
"But suppose it's a little girl - that brings it's out its own, slightly different attraction to it."
Rachel Hughes runs the group Mums of Little Ones at HTB church in London.
She told Premier a second baby can often be harder than the first.
"When we went from one to two we realised that we were almost outnumbered.
"That Saturday morning lie in was game over, we were totally all hands on deck all the time," she said.
Prime Minister David Cameron tweeted he was "delighted by the the happy news that they're expecting another baby".
Clarence House said: "Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their second child."
Kate and William already have one son, George, who is one year old.
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