Already parents to Prince George, four, and Princess Charlotte, two, speculation had been rife about whether the couple would add to their family any further, and now that the Duchess is pregnant again, there's an incredible amount of interest in what the new addition will mean for the future of the throne.
As far as we know, Kate is carrying one child, and there is no history of twins on either side that would suggest anything otherwise. But some observers have still found themselves wondering: what would happen if the couple had twins?
Well, for one thing, they'd certainly have their hands full, but on a more official level, the birth order of the twins would be very significant.
If William and Kate had had twins before 2012, and they had been a boy and girl, the boy would automatically have been the next in line to the throne, behind grandfather Charles, and father William.
However, thanks to a change in the law made in recent years, any twins born after this time would have an equal chance of wearing the crown, regardless of their gender - the spot would simply go to the first born child. For the same reason, they would also remain behind big brother George and big sister Charlotte, whereas before, she would be have taken a step back in favour of a son.
When the next (presumably singular) royal baby arrived, he or she will be fifth in line to the throne, pushing uncle Prince Harry down to sixth place.
And if twins did ever pop up in the family? According to reports, no mother of a future monarch has given birth to twins in Britain since the 15th century – so those babies would well and truly be making history.