For this very modern couple, yesterday’s news is further proof of their defiance of recent royal convention and a determination to fashion their lives in the middle-class Middleton way.
Not since the Queen, who had four children, has a royal prince had more than one sibling. Prince Charles, Prince Andrew, Prince Edward and Princess Anne each had only two children.
So not for the first time, William might seem to be following the example of his grandmother rather than his father when it comes to parenting.
He admires the conscientious and diplomatic way she has gone about her role as sovereign, a very different style from that of his father, whose views on so many subjects have been broadcast far and wide.
Equally, the Queen is said to see in William a future monarch who will reign in a style not dissimilar to her own.
For this very modern couple, yesterday’s news is further proof of their defiance of recent royal convention and a determination to fashion their lives in the middle-class Middleton way. Handful: Princess Anne with Peter and newborn Zara in 1981
Not since the Queen, who had four children, has a royal prince had more than one sibling. Prince Charles, Prince Andrew, Prince Edward and Princess Anne each had only two children. Heirs and a spare: Charles with a young William and Harry
‘William is much more like his grandmother than his father in the way he is so much less fussy and extravagant than the Prince of Wales – and that suits her own understated tastes far better,’ says a figure close to the Royal Family.
But it would be wrong to think that this was all just about William following in the footsteps of his grandmother’s generation.
For her part, Kate is the product of a three-child family and the closeness between her, as the eldest, and her sister Pippa and brother James undoubtedly appealed to William.
In the early days of their romance, the prince first discovered the joys of visiting the Middleton family home at Bucklebury, Berkshire, where everyone mucked in and did things together. This was so different from his own experience growing up with a mother and father leading separate lives in separate homes.
Now that William and Kate have reached this third child mile-stone, it will be fascinating over the coming years to see how this future monarch and his queen will combine the heavy demands of duty with those of a very young family.
No royal couple has ever had such a challenging future – maintaining historical tradition while being very much part of a fast moving, modern world.
Having finished his job as an air ambulance pilot and with his only role now as a full-time royal, William will be under pressure not to sacrifice his official duties in favour of time in the nursery. In other words no paternity leave – or at least a short one.
One old friend says: ‘It wouldn’t surprise me if Kate’s attitude to William is: “Look, let me get on with the family and you do the work.” Although, because of her illness, he will want to make himself available for her.’
There is also already debate at how William and Kate’s news might affect Prince Charles’s reported desire to slim down the Royal Family. Two of a kind: Andrew with Beatrice and Eugenie in 1997
Another knock-on effect will inevitably involve Prince Harry.
Not only will the arrival of a third niece or nephew push him further down the line of succession to number six, he will hope it may also distract attention from his deepening relationship with American actress Meghan Markle.
There is also already debate at how William and Kate’s news might affect Prince Charles’s reported desire to slim down the Royal Family. ‘If anything, the duchess’s pregnancy will actually speed matters up in that area,’ says a courtier intriguingly.
‘When Charles comes to the throne, the monarchy will be concentrated on him, his sons and their children. Figures such as Prince Andrew and his daughters will occupy peripheral roles at best.’
By most standards of modern relationships, William and Kate’s progression from dating to parenting has been leisurely.
By the time she had given birth to Prince George, Kate had been with William for 12 years. Such slow progress wasn’t exactly the rush of blue blood normally associated with royal marriages.
This week, Prince George starts at school in London, the major event which lay behind his parents’ decision to give up their life in rural Norfolk and move back to Kensington Palace. Royal family: Edward and wife Sophie with Louise and James
Kate had her first baby at 31. At that age, William’s mother, Princess Diana, had separated from Charles and was making a new life for herself. Indeed, poignantly, by the time Kate gives birth, she will be 36, the age at which Diana, tragically, lost her life.
William also knows how much Diana longed for a third child, particularly a daughter that she hoped would restore her family happiness.
It is hard to think that, back when William chose as his bride a middle-class girl, some doubted the match and cruelly mocked the fact her mother was a former air hostess.
The truth is that Kate has risen brilliantly to the challenge.
Those cynics were silenced by the Queen, whose fondness for William and Kate is such that she has frequently included the Middleton family in domestic royal gatherings.
Meanwhile, the couple have to adapt their daily routine to manage their new life.
This week, Prince George starts at school in London, the major event which lay behind his parents’ decision to give up their life in rural Norfolk and move back to Kensington Palace.
It seems unlikely that they will uproot their family again to return to the countryside for the new baby’s early years, however much they may yearn to do so.
The Queen’s father, George VI, was one of six and Queen Victoria had nine children. In 1968: Princess Margaret holding Sarah and David
And then there is the question of how William himself will deal with the new arrival. Two years ago he took six weeks paternity leave from his job as an air ambulance pilot – and was apparently surprised by the criticism he received.
Now, he will be under pressure not to sacrifice his official duties in favour of time in the nursery.
Indeed, many of us were surprised that the couple’s news came just days after palace officials had revealed details of the Cambridges’ autumn schedule, which is thought to include a short tour to Scandinavia.
‘The couple have always put family first and it will be interesting to see how they cope with all the new demands of being working royals and having three children under six,’ says a long-term courtier. They are fortunate that they have the full support of the Queen.’
It will also be interesting to see how much help they plan to engage. The couple have been hands-on as far as possible with their children, employing a single nanny and minimum domestic staff.
It has helped, of course, that Kate’s capable mother has also been hugely involved. As a mother of three, she remains a powerful influence over Kate’s choices for her family.
Kate had her first baby at 31. At that age, William’s mother, Princess Diana, had separated from Charles and was making a new life for herself
While Carole and Michael Middleton have provided a precedent for having three children, on the royal side, apart from the Queen, you have to go back before the Second World War to find a large family.
The Queen’s father, George VI, was one of six and Queen Victoria had nine children.
For Kate, pregnancy third time round should hold no real fears. True, there is the toll the pregnancy will take on her physically but she know that, previously, the chronic morning sickness in the early weeks was followed by boundless energy later.
She knows what to expect on a wider level, too. She realises that the arrival of a second son or daughter will trigger an avalanche of public affection — and that the joy of anticipation will be shared by millions around the world.
She will remember fondly how it was William who swept aside royal protocol to allow her to adjust to pregnancy and mother-hood with a degree of privacy that was missing when Princess Diana gave birth to him and his brother Harry.
And she will be sure he does the same again this time for her and her new prince or princess.