Where will the baby fall in the line of succession?
A great-grandchild of the Queen and the Duke of Edinburgh, he or she will be born fifth in line to the throne, bumping uncle Prince Harry out of the top five into sixth place.
Will the royal baby be a boy or a girl?The Prince of Wales is first in line, followed by William who is second in line, Prince George third and Princess Charlotte fourth.
Younger male siblings are no longer allowed to jump ahead of their older sisters in the line of succession.
Will he or she ever be monarch?
Unlikely. It's rare, but not unheard of, for a third-born royal in direct succession to become king or queen.
This Cambridge sibling will live their life happy in the knowledge they will not be expected to wear the crown, leaving that role to older brother George.
William IV, a Hanoverian king who ruled from 1830 to 1837, was a third child - of George III and Queen Charlotte.
He acceded to the throne when he outlived his older brothers, George IV, who died without an heir, and Frederick, Duke of York.
What will the baby be called?
After picking the regal George and Charlotte for their first two, William and Kate will not be expected to suddenly choose an unusual name for their third.
Alice, Mary, Alexandra or Victoria for a girl, or Frederick, James, Arthur or Philip for a boy are good bets.
Will they be a prince or princess?
Yes. The new baby will be an HRH and a Prince or Princess of Cambridge - thanks to the Queen who stepped in ahead of Prince George's birth to ensure all William's children had fitting titles.
Will they hire a second nanny?
Kate and William already have the help of George and Charlotte's full-time live-in nanny Maria Teresa Turrion Borrallo.
But Maria could be given a helper - perhaps a fellow Norland nanny - as she cares for the three youngsters.
William and Harry had two much-loved nannies at the same time when they were young - Barbara Barnes and her deputy Olga Powell, who later took the lead role.
When and where will the baby be born?
With Kate experiencing two successful, straightforward deliveries at the Lindo Wing in St Mary's Hospital, Paddington, London, she will head back to the exclusive private delivery suite for the arrival of her third child.
Charlotte's birth appeared to be much quicker than George's so there is always the outside chance this baby could make a speedy, surprise arrival at home.
Kensington Palace has only said the baby is due sometime in April. Suggestions have included St George's Day - April 23.
Looking at when Kate went on maternity leave last time and when her babies arrived, the baby could arrive as late as April 27 or April 28.
Will there be the same level of baby fever?
Although this baby will only be fifth in line, the popularity of William, Kate, George and Charlotte across the globe means the arrival of the newest member of the Windsors will be high-profile once again.
In May 2015 ahead of Charlotte's birth, scores of foreign media outlets descended on the Lindo Wing. Her birth dominated the news agenda in the UK and hit the headlines around the world.
Will they stop at three?
Who knows? The Queen and the Duke of Edinburgh had four children and the Queen was 37 when she had her youngest, Prince Edward.
Kate is only 36 so has time to even out the numbers.
William's great-great-great-great-grandmother Queen Victoria had nine children.