A statement from Kensington Palace announcing the pregnancy said the Duchess was unable to carry out her planned engagement at the Hornsey Road Children's Centre in London today and is being cared for at the Palace.
There will soon be another Royal baby!
According to the statement, Queen Elizabeth and members of both families are delighted with the news.
The Duchess experienced hyperemesis gravidarum during her previous pregnancies - an extreme form of morning sickness that can lead to constant vomiting, weight loss, dehydration and depression.
Kate Middelton and Prince Williams announced they are expecting another child.
Kate and William married in a lavish ceremony in 2011 watched by about two billion people around the world.
"Given the recent anniversary, there'll be plenty of interest in the name Diana if the baby is a girl", the agency said. Royal watchers recorded every aside for clues as to what the plans might be.
Kate is the eldest of three siblings, and reportedly had a very happy childhood.
And Kate also gave an extremely telling clue that she wants more children in the future in Poland.
Saying thank you for the present, she turned to the Duke, laughing, and said: 'We will just have to have more babies'.
The unborn child will become fifth in line to the throne.