Kate would have not long since given birth to her baby with the maximum time between the birth and the wedding to be eight weeks.
So would the newborn attend the biggest wedding of the year?
Prince George, 4, and Princess Charlotte, 2, may well play a part in the wedding itself.
Royal weddings tend to feature children as attendants, so George would be a pageboy and Charlotte a flower girl.
But there would be no such role for a tiny baby – and Kate is unlikely to bring her newborn for one particular reason.
As the child may not have been seen in public much yet, Kate will not want to take attention from Harry and Meghan on their special day.
The Royal Family are notoriously careful about sharing the spotlight, and not having one particular member outshine the others.
Harry and Meghan are thought to be choosing a wedding date with Kate’s birth in mind, consciously not wanting to overshadow her.
And the couple went to great lengths to make sure Meghan did not overshadow Pippa at her wedding to James Matthews earlier this year, with her entrance not captured on camera.
If Kate and William do decide not to bring their third child along to the wedding, there is one place they are likely to leave it.
Kate’s parents, Carole and Michael, have looked after their other two children many times, with Kate a regular at their Berkshire home.
So potentially, they could watch their new grandchild while Kate and William enjoy a day in London for Harry’s wedding.
Kate and William have yet to reveal the sex of their unborn child, or what they will be naming it.
But the potential names they could choose have been revealed – will Kate and William name their child something classic?