Infant will shake up 300 years of royal history
KAte and William's baby will ultimately accede to the throne regardless of its sex after changes in the rules of royal succession.
In a break with more than 300 years of English constitutional tradition, laws that would have passed the crown to the oldest male heir of the Duke of Cambridge will not apply.
And there were reports yesterday that even Queen Elizabeth herself did not know about the pregnancy and was only told because Kate had to be rushed to hospital due to her morning sickness.
Under the previous rules, any male child born to the couple would have taken precedence in the order of succession over older sisters.
That would have meant that were next year's child to be female, her place in the line of succession could have been taken by a younger brother.
The laws required to change the succession rules have not yet been passed, but ministers insist that a political agreement David Cameron made with other Commonwealth leaders last year is enough.
At a Commonwealth summit in Perth, Australia, Mr Cameron last year agreed in principle that succession rules will be changed across the Queen's Realms, the 16 nations where the queen is head of state.
Nick Clegg, the deputy prime minister, last month told MPs that there has been a "de facto" change in the succession rules.
"The change to the rule came into effect from the point of the Perth conference last year," he said. "A de facto change has already been introduced pending the legal changes that now need to be made."
Ms Middleton's extreme morning sickness has given rise to speculation that she may be carrying twins, since hyperemesis gravidarum is associated with multiple births.
Twins have never been born into prominent positions in the British line of succession before.
However, were she to bear two – or more – children next year, the first to be born would take a higher place in the line.
Last year, Crown Princess Mary of Denmark gave birth to twins, a boy and a girl.
Before the birth, there was speculation that Princess Mary would have a Caesarean delivery, leading to reports that the presiding doctor would effectively be choosing the babies' position in the line of succession.
However, the babies were delivered without surgery. The boy, Prince Vincent, was delivered around 25 minutes before his sister, Princess Josephine.
The law on the royal succession in Britain is often referred to as the Act of Settlement, but the rules are actually set down in several different pieces of legislation passed in the 17th and 18th Centuries. They include the Act of Settlement, the Bill of Rights, the Royal Marriages Act and Princess Sophia's Precedence Act.
William and Kate will do their utmost to give their son or daughter as normal an upbringing as possible. William has often spoken of how important it was for him to be treated like everyone else.
He relished the time he spent away from the media at university and in the armed forces.
Kate too, although also from a wealthy background, has been praised for her down-to-earth approach. The youngster will, however, grow up, just as William did, with the knowledge that they will one day wear the crown.
A private education beckons and possibly a stint at boarding school. William went to Eton and Kate to Marlborough College in Wiltshire.
The child's milestones – from their first day at school to their first public engagement – may well be recorded by the media and a barrage of photographers just like William experienced.
Yet William and Kate could decide to keep all such things private in a bid to offer as much protection to their child as possible.
Their recent experience will have made them all the more wary of guarding their private lives. Kate was pictured topless while sunbathing on holiday in a remote French chateau after being snapped by a photographer using a long lens.
Summer holidays will often be spent with the family in the Scottish countryside up at Balmoral.
Christmases will partly be at Sandringham in Norfolk with the royals, and trips to Buckingham Palace and Windsor Castle will expose the youngsters to opulent surroundings. (© Daily Telegraph, London)