Kensington palace has announced that - as when the duchess was pregnant with Prince George - she was suffering from very acute morning sickness Hyperemesis gravidarum and was being treated by doctors at the palace.

Prince William and Catherine's second baby will become the
fourth in line to the throne, behind older brother Prince George, who is 13 months old now.
The palace said the Queen and both families were delighted with the news.
       This makes it the second time, William and Kate have been forced to announce a pregnancy before the duchess passed the significant 12-week milestone because Kate is suffering from an acute form of morning sickness - though this time she's being treated behind palace walls and not at a private hospital with representatives of the world's media trooping outside.

The pregnancy has generated happiness internationally and will continue to do so but this impending birth will lack the constitutional significance of Prince George's arrival.
As things stand, the as yet unborn boy or girl isn't destined to be a monarch.
           She or he will occupy the same position as Prince Harry once did. His mother, Diana, used to call him "the back up".
Monday's announcement has performed one unintended but useful role for the Windsors. Headlines about the Queen's view of Scottish independence will be replaced by extensive coverage of a royal birth next year.
     The baby, expected to be born in the spring, will be a younger sibling to 1-year-old Prince George. He or she will be fourth in line to the British throne after grandfather Prince Charles, father Prince William and big brother George.