London - For this very modern couple, Monday'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.
"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 milestone, 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."
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.