IT is no secret that the Duke and Duchess of Cambridge dreamed of starting a family of their own.
Before their wedding, they both spoke earnestly about the next stage in their life.
Kate, who is close to her own parents and siblings, revealed: “I hope we will be able to have a happy family ourselves.”
William too expressed his desire to have children with his new wife.
There was speculation that the Duchess, who showed herself to be at ease with youngsters on official engagements, was broody.
Even the food they had shipped to their honeymoon island in the Seychelles - apparently Brussels sprouts - led to suggestion Kate was on a fertility diet aimed at boosting her intake of folic acid.
There will be much excitement among the couple’s families.
The baby will be Carole and Michael Middleton’s first grandchild and the first for the Prince of Wales - although he is already a doting grandpa to the Duchess of Cornwall’s grandchildren.
Yet as well as the personal joy it will bring, William and Kate’s child will have a wider historical and constitutional impact: the new baby symbolises the continuation of the monarchy.
He or she is destined to be king or queen and will be born third in line and in direct succession to the throne.
The child will one day be head of the armed forces, supreme governor of the Church of England and head of the Commonwealth, which covers 54 nations across the world, and subsequently head of state of 16 countries.
In blunt terms, Kate will be fulfilling one of her essential duties as a royal wife by producing an heir.
The birth will also expose the Cambridges to a new level of public fascination.
With the world ready to watch their baby grow up, William and Kate will have to balance protecting their prince of princess with the nation’s avid interest in its future king or queen.
When William was born, thousands of people gathered outside Buckingham Palace to wait for the birth to be formally announced.
He was the first future British king to be born in a hospital, delivered at the private Lindo Wing of St Mary’s Hospital in Paddington, London.
The newborn Prince arrived in June 1982 just under a year after his parents’ marriage, while the Queen had Charles in a similar timescale.
When the Queen was born in her grandparents’ London home in Mayfair, the home secretary Sir William Joynson-Hicks waited in the next room as part of an age-old custom designed to prevent a substitute baby being smuggled in.
Kate will luckily not have to suffer such ignominy.
The birth of the Queen’s cousin Princess Alexandra in 1936 was the last occasion on which the home secretary was present.
King George VI declared that a minister was needed only for those in the direct line of succession, but by time Charles was born in 1948, the practice had been abandoned after being deemed “neither a statutory requirement nor a constitutional necessity”.
Kate will be tended by a top medical team. Royal mothers are usually looked after by the Queen’s gynaecologist - currently Alan Farthing, the former fiance of murdered TV presenter Jill Dando.
The birth of the prince or princesses comes following a radical shake-up of the monarchy’s successions rights.
Prime Minister David Cameron announced in October last year that the 15 other Commonwealth countries where the Queen is head of state had agreed to give female royals the same rights of succession as their brothers.
“Put simply, if the Duke and Duchess of Cambridge were to have a little girl, that girl would one day be our queen,” Mr Cameron said.
When the law goes through, first-born royal daughters in direct line to the throne will no longer be leapfrogged by their younger male siblings.
The proposed constitutional change was spurred by the marriage of the Duke and Duchess of Cambridge in anticipation that they would produce a child.