With Prince George and Princess Charlotte, Kate gave birth just over five weeks after going on maternity leave. She went on maternity leave for her third child on March 22, so this suggests the baby might put in an appearance around April 27 or 28.
Both Prince George, four, and Princess Charlotte, two, are thought to have been born slightly late: in 2015 the Duchess of Cambridge let slip that her due date was towards the end of April, however, Princess Charlotte was born on 2 May. This might mean another longer-than-average pregnancy is on the cards for the Duchess.
The new arrival will come just weeks before Prince Harry and Meghan Markle tie the knot on May 19, hopefully giving Kate a few days to recover.
Where will the royal baby be born?
Parking restrictions are in place outside the Lindo Wing of St Mary's Hospital in London in preparation for the birth.
Crowd barriers are set up on the street outside to accommodate the television crews, journalists and photographers who will be present as she goes into labour.
Both Prince George and Princess Charlotte were born at the Lindo Wing.
The Telegraph's Anna Maxted had all three of her children there, and describes the rooms as basic, but the care as first-class.
When the Duchess of Cambridge was pregnant with Princess Charlotte in 2015, contingency plans were put in place. If the Duchess unexpectedly went into labour while visiting her parents in Bucklebury, she would have been taken to the Royal Berkshire Hospital in Reading, while if she went into labour at Anmer Hall, the couple's country residence in Norfolk, she would have given birth at the Queen Elizabeth Hospital in King's Lynn.
What names could the Duke and Duchess of Cambridge be considering?
The Duke and Duchess have chosen solidly traditional royal names for their children in the past, making it likely they will reference previous monarchs this time as well.
Alice, Mary, Alexandra or Victoria for a girl, or Frederick, James, Arthur or Philip for a boy are good bets.
There is one former king who's highly unlikely to ever have another royal namesake: King John. That's because in his 17-year reign he managed to so sully his name it's pretty much been removed from royal circulation.
Not only was he exceptionally rapacious and cruel (he had a penchant for starving his enemies to death), John was also one of the most cowardly and incompetent scoundrels to sit on the throne.
Why did they pick George and Charlotte?
The couple's first child George will in time become King George VII, following a tradition dating back to German-born George I, the first Hanoverian king of Great Britain who acceded to the throne in 1714. The last was the Queen's father George VI who died in 1952 but was known to his family as Bertie.
Among the previous monarchs to have had the name is George III - on the throne from 1760 to 1820 - who is predominantly remembered for losing the American colonies and his sanity. However, the Prince of Wales has previously disclosed that he is the king he most respects, describing him as a good man who was simply misunderstood.
The Duke and Duchess of Cambridge honoured both royal tradition and family ties by naming their second child Charlotte.
It won't be long until baby number three joins the Duke and Duchess. Photo / Getty Images
The name's royal pedigree includes Queen Charlotte, the wife of George III, who bore him 15 children and helped found Kew Gardens. Queen Charlotte's & Chelsea Hospital, which is named after her, is part of the same NHS Trust as St Mary's Hospital Paddington, where the private Lindo Wing is situated.
Her granddaughter Princess Charlotte of Wales, who married on May 2, the day the new Princess Charlotte was born, died in childbirth in 1817 at the age of just 21. She was the only child of George, Prince of Wales, who would go on to become King George IV.
Charlotte also happens to be the middle name of the Duchess's sister, Pippa Middleton, and goes back in her family to her paternal great-great-great-grandmother Charlotte Ablett, born in 1825 and therefore possibly named for Princess Charlotte.
How will the third royal baby affect the succession?
The new royal baby will be fifth in line to the throne, behind the Prince of Wales, the Duke of Cambridge, Prince George and Princess Charlotte. It means Prince Harry will drop down to sixth in line, and every other member of the Royal family will be one step further away from the throne.
Because of recent changes to the laws governing succession, the baby's sex will make no difference, as boys no longer have precedence over girls.
If the change hadn't been made and the third baby were a boy, he would be fifth in line instead of Princess Charlotte - however that will now not be the case.
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 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.
Will the Duchess of Cambridge have a natural birth?
Both Prince George and Princess Charlotte are believed to have been born by a natural delivery, meaning the Duchess of Cambridge will presumably be planning to have a natural birth this time as well.
In 2015 Pat O'Brien, a consultant obstetrician at University College Hospital, said that because of the speed with which the Princess Charlotte was delivered after the Duchess was admitted to hospital, it is likely that she went into labour naturally.
Kate is due to give birth some time in April. Photo / Getty Images
He said: "The fact that she went into hospital at around 6am and had the baby by 8.34am would not imply that she was induced at all.
"Second time labours tend to be quicker than first time labours. Everything tends to go faster. The dilation stage is quicker, the pushing stage is usually quicker and easier as well, and the baby comes out more quickly. You would expect it to be an easier labour anyway."
Who will deliver the royal baby?
The medical team in charge of safely delivering Princess Charlotte was led by Guy Thorpe-Beeston, Surgeon Gynaecologist to the Royal Household.
Mr Thorpe-Beeston, an obstetrician who specialises in high-risk pregnancies, has a private Harley Street practice and is a consultant at the Portland private hospital and at the Chelsea and Westminster Hospital in the NHS.
A Cambridge graduate, he worked at King's College Hospital in London before becoming a consultant in foetal medicine at Chelsea and Westminster in 1996.
He was assisted by Alan Farthing, Surgeon Gynaecologist to the Queen, who was the fiance of the television presenter Jill Dando at the time of her murder in 1999, and has been part of the Queen's medical household since 2008.
In 2014 Sir Marcus Setchell, the recently retired surgeon gynaecologist to the Queen who delivered Prince George, told BBC Radio 4's Woman's Hour: "What happened in labour is an entirely private matter, but I do think there are certain situations when someone is giving birth that it's important…to have a specialist…in the same room to deal with anything that's immediately going to be wrong."
The age-old custom behind announcing a royal baby
After the Queen, other members of the Royal family and the Middleton family have been told the good news, the royal communications team will email media organisations with the sex, weight and timing of the birth.
Moments later the news will be announced via royal Twitter and Instagram accounts before a traditional paper announcement is placed on an easel outside Buckingham Palace.
The age-old custom of placing a paper proclamation for the public to see at the Queen's London residence took place when Prince George and Princess Charlotte were born.
A brief formal announcement - on foolscap-sized paper set in a dark wooden frame - will be placed on an ornate golden easel on the forecourt of the Palace.
It will confirm the sex of the baby and the time of birth, but will give little else away, other than usually revealing that the baby has been "safely delivered" and mother and child are "doing well".
It used to be hand-written, but is now typed. It will also be signed at the hospital by the doctors who tended to the duchess, and then ferried back to the Palace by car.
Her Royal Highness The Duchess of Cambridge was safely delivered of a daughter at 8.34am.
— The Duke and Duchess of Cambridge (@KensingtonRoyal) May 2, 2015
When will we see the royal baby?
After the birth of Prince George, the Duchess left hospital the next day, while she left the hospital to go home on the same day after giving birth to Princess Charlotte.
But the length of the Duchess's hospital stay this time around will be entirely down to how she is feeling, so there is no guarantee we will see the baby within 24 hours of its birth.
The Duke and Duchess are expected to leave the Lindo Wing by the front door to give the waiting media the chance to take the photographs that will no doubt adorn the front pages of newspapers the next day.
After the birth of Princess Charlotte, the Duke and Duchess spent the first nights of her life at Kensington Palace before driving north to their country home, Anmer Hall, on the Queen's Sandringham estate, where they were likely joined there by the Duchess's parents, Michael and Carole Middleton, who often stayed over to help look after Prince George.
However, the family permanently relocated to Kensington Palace last year, after deciding Prince George should be schooled in London, so it's unlikely that they'll head back to Norfolk for a while.
Catherine carrying a newborn Princess Charlotte. Photo / Getty Images
How soon will we know the name?
The names given to royal babies are not usually revealed straight away, and the public is often left guessing for several days.
William and Kate took two days to announce both George and Charlotte's names, informing the Queen of their choice beforehand.
Mike and Zara Tindall used Twitter to unveil their daughter's name six days after she was born in 2014, with proud dad Mike tweeting: "For everyone who has asked what our daughter's name is, it's Mia Grace Tindall."
When Princess Beatrice was born in 1988, it was two weeks before her name was known.
In 1982, the Prince and Princess of Wales waited seven days before deciding upon and announcing Prince William's name.
The Prince of Wales's name, however, remained a mystery for an entire month and was only declared ahead of his christening in the Music Room of Buckingham Palace in December 1948.
Will the Duke and Duchess take maternity and paternity leave?
They have on previous occasions: after the birth of Princess Charlotte, the Duke took two weeks' statutory paternity leave from his job as an air ambulance pilot, which he quit in July 2017 to become a full time royal, and the Duchess took maternity leave from her official royal duties.
After the birth of Prince George, the Duchess was back in action after five weeks, joining her husband to start the Anglesey Coastal Ultra Marathon. However, she maintained a light workload for several months after both births.
Will the Duchess hire a maternity nurse?
As well as having a full-time nanny and plenty of help from the Middletons in 2015, the Duke and Duchess of Cambridge hired another temporary nanny to help them through the first weeks of Princess Charlotte's life.
The couple were determined to manage without professional help when Prince George was born, but he was not a good sleeper and the Duchess became exhausted after countless sleepless nights.
They eventually accepted they needed help, and hired Spaniard Maria Borrallo from the world-famous Norland nanny agency when Prince George was seven months old.
When Princess Charlotte was born they reportedly made arrangements through Norland for a maternity nurse to work full-time for at least three months, to get them through the most difficult period of their new baby's life.
Will there be royal baby souvenirs?
Almost certainly. Perhaps the most tasteful souvenir will be a commemorative £5 coin that is likely to be a hit with collectors.
