Poor Kate is laid up in hospital with hyperemesis gravidarum - extreme morning sickness - which causes dehydration.
The condition affects up to one in 50 women, and can be more common when the mum-to-be is expecting twins, due to all those lovely hormones racing around the body.
So, could there be two royal bundles of joy for Kate and Wills come summer 2013?
The ancient (and rather sexist) rule of royal primogeniture, which gave males precedence to the top job, was ditched last year, meaning Kate and Wills' first born - no matter whether it's a little girl or boy - will become third in line to the throne, behind Charles and William.
If the royal couple do welcome twins, the firstborn, no matter what the sex, will take the crown.
Twins have not been born into prominent positions within the British line of succession before.
It is understood Kate is has not yet reached the 12 week mark, and the pregnancy announcement was made because the Duchess was admitted to the King Edward VII Hospital in central London with acute morning sickness.
William was by his wife's side yesterday, and left the hospital around 8.20pm. Kate is expected to stay in hospital for several days.
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