Prince Harry and Meghan Markle's first-born child would become seventh in line to the throne, behind their father – but only if the Duke and Duchess of Cambridge have no more children.
The royal couple – who tie the knot in Windsor on Saturday – are said to be keen to start a family soon after their wedding.
But how would their future child affect the royal line of succession? And who would move down?
Last month, the birth of Prince Louis made history when he slotted fifth-in-line to the throne.
Prince William and his wife Kate's third child means the new line of succession will see the new Prince of Cambridge follow the Prince of Wales, Duke of Cambridge, Prince George and Princess Charlotte.
Historically, Princess Charlotte would have lost her spot to her younger brother, but new legislation introduced in 2013 overhauled the order of succession.
Prince Harry now sits in sixth place, while Prince Andrew was pushed down to seventh-in-line, meaning he will not have to ask his mother's permission if he wanted to re-marry.
It is at this point that any future children of Harry and Meghan could disrupt the line.
Their first-born would take the position of seventh-in-line, bumping great-uncle Andrew down one spot. If the couple go on to have any more children, these would push the Duke of York even further down the line.
However, this all changes if the Duke and Duchess of Cambridge go on to have more children. Any future children of William and Kate would assume their position after Prince Louis, meaning Harry and his future offspring move down.
As the line changes with birth, it also changes with death. Should any senior royal in line to the throne lose their life, those beneath them move up.
According to the new legislation, the first six people in the line of succession must obtain the Queen's permission to marry.
Previously, under a law dating back to 1772, all descendants of George II had to seek the monarch's permission.
The birth of a new baby for the Cambridges means the Duke of York can technically marry without the express permission of his mother.
Line of succession
The Queen, who turned 92 this weekend, is Britain's longest reigning monarch and has ruled over us for 65 years.
As the child of a royal "spare," the Queen, was thought unlikely to reign until her uncle, King Edward VIII, abdicated in favour of her father in 1936, whereupon he became King George VI and she the heir to the throne.
Elizabeth became Queen upon his death on February 6 1952. But what happens when she dies and who will replace her? Here's the line of succession as it stands today.
1. The Prince of Wales
Heir to the throne and the next king of the United Kingdom and the Commonwealth Realms. The Prince of Wales, 69, already holds the record for the longest-serving heir apparent in British history, having been in the position since 1952. He will also be the oldest person to be crowned in British history when he eventually succeeds the Queen.
2. The Duke of Cambridge
As the eldest son of the heir, the Duke of Cambridge will one day succeed his father the Prince of Wales. At the age of 35, he may have 30 or so years to wait, if his father enjoys the longevity of other members of the Royal family.
3. Prince George
The Duke and Duchess of Cambridge's first-born son, four, will one day be King George VII, meaning that anyone alive today is unlikely to see another Queen on the throne in their lifetime once Elizabeth II's reign is over.
4. Princess Charlotte
Prince George's two-year-old sister is the "spare to the heir", and will face the same problems as Prince Harry and the Duke of York in trying to find the right role for themselves during a lifetime constrained by their royal title. She would only become queen if Prince George predeceased her without having any living children.
Changes in the laws of succession in 2013 mean the sex of the baby will be irrelevant to his or her place in the succession. Princess Charlotte is the first royal princess to be born within the direct line of succession since the Princess Royal in 1950, and is likely to one day inherit the title, which is traditionally given to the oldest daughter of the monarch.
5. Prince Louis
The birth of the Duke and Duchess of Cambridge's third child changes the line of succession again: Prince George and Princess Charlotte's new baby brother is now fifth in line to the throne, bumping uncle Harry out of the top five and into sixth place.
6. Prince Harry
The all-action Prince, 33, has spent most of his life being the third in line to the throne, behind his brother Prince William, but has now found himself being bumped down a place to sixth. He is due to marry American actress Meghan Markle in just a few weeks.
7. The Duke of York
Once second in line to the throne, Prince Andrew, 57, is now so far down the list that he is no longer part of the "core" Royal family that will share the burden of official engagements in the future. His daughters, Princesses Beatrice and Eugenie, are no longer entitled to police bodyguards, which now have to be funded by Prince Andrew.
8. Princess Beatrice
Princess Beatrice has struggled desperately with the problem of how to combine a royal title with a need to find work. After quitting her £20,000-a-year job with Sony Entertainment Pictures in 2014, she treated herself to four holidays in the space of a little over a month.
The 29-year-old, who was the first member of the Royal Family to run a marathon in 2010, is now pursuing a business career five years after graduating in history and the history of ideas at Goldsmiths College, London, as well as continuing with charity work. She and her long-term former boyfriend Dave Clark called time on their 10 year relationship in summer 2016.
9. Princess Eugenie
The Duke of York's younger daughter, who is 27, moved to New York to work as an auctions manager in 2014 after taking a degree in English Literature and History of Art at Newcastle University. In 2015, she started working for Hauser and Wirth art gallery in London as an associate director. In January, it was announced that Eugenie would marry her long-term boyfriend Jack Brooksbank, in Windsor, in October.
10. The Earl of Wessex
Having failed to complete his Royal Marines commando training, the Earl, 53, became a case study in the pitfalls of trying to earn a living when your mother is the Queen. He organised the cringe-making Grand Knockout Tournament (better known as It's a Royal Knockout) in 1987, then carried on with his TV production career, but incurred the fury of his brother the Prince of Wales when a camera crew employed by him unwittingly broke the terms of an agreement to leave Prince William alone during his time at university. He is now a full-time working royal. His nine-year-old son Viscount Severn is 10th in line to the throne, with 13-year-old daughter Lady Louise Windsor 11th.
13. The Princess Royal
Because the 2011 law change on the succession did not apply retrospectively, the Princess Royal, 67, remains behind her three brothers in the line of succession despite being the Queen's second-born child.
Often described as the hardest-working member of the Royal family, she carries out hundreds of official engagements each year without receiving much coverage in the national media.
Her son Peter Phillips, 40, is 13th in line, with his children Savannah and Isla 14th and 15th. The Princess's pregnant daughter Zara Tindall, 36, is 16th in line, and her three-year-old daughter Mia is 17th.