The Duke and Duchess of Sussex's son will be seventh-in-line to the British throne.
He will come behind his father Prince Harry in the line of succession, bumping his uncle, the Duke of York, further down the line into eighth place.
The Duchess of Sussex gave birth to Baby Sussex early on Monday morning UK time, after months of speculation about her exact due date.
"Her Royal Highness The Duchess of Sussex was safely delivered of a son at 0526hrs (2.26pm Monday AEST)," Buckingham Palace said in a statement.
However, he will not be given the title of prince unless the Queen steps in to change the rules. Meghan and Harry's son will instead assume the title of lord, not prince.
They can thank King George V for that, The Washington Post reports.
Back in 1917, William and Harry's great-great-grandfather decided to limit the titles of members of the royal family, stating: "The grandchildren of the sons of any such Sovereign in the direct male line (save only the eldest living son of the eldest son of the Prince of Wales) shall have and enjoy in all occasions the style and title enjoyed by the children of Dukes of these Our Realms."
This means that as Meghan and Harry's child will be a great-grandchild of the sovereign (at least until Charles inherits the throne) he or she will be placed too far down the line of succession to rightfully assume the title of prince or princess. It is believed that any children of the duke and duchess of Sussex will be known as Lord or Lady Mountbatten-Windsor, unless the queen steps in to change that.
In 2015, the queen opted to make Prince George a prince and went on to do so with his younger siblings, Princess Charlotte and Prince Louis. "The queen might well intervene in this case," royal commentator Richard Fitzwilliams said.
There's also speculation that Meghan and Harry may prefer that their children do not adopt the title of HRH in the hope that they will lead a more normal life out of the intense royal spotlight.
As for two other common questions, let's take a look:
Will Meghan and Harry's child ever occupy the British throne?
Gender no longer plays a factor in determining who is next in the line of succession. Male bias was removed in the Succession to the Crown Act, which was passed in 2013. Nowadays, the order in which members take the throne is based on age.
As of today, the line of succession to the British throne looks like this:
Next in line behind his mother, reigning monarch Queen Elizabeth II, is Prince Charles, followed by his eldest son, Prince William. After Prince William are his three children: Prince George, Princess Charlotte and Prince Louis. Behind Prince Louis is his uncle, Prince Harry, at No. 6.
Today's announcement puts Harry and Meghan's baby in seventh place, which makes it highly unlikely but not impossible for their child to one day be king.
In eighth, ninth and 10th place are the queen's second son, Prince Andrew, and his two daughters, Princess Beatrice and Princess Eugenie.
What will the baby's name be?
Your guess is as good as ours.
Meghan and Harry are well known for their ability to shake up the somewhat stale traditions of the royal family. As Harry is not directly in line to the throne, it's often thought that he has more freedom to stray from royal norms when it comes to living his life. In their wedding earlier this year, the couple followed some but also jettisoned some age-old traditions — and were widely praised for doing so.
Some think the couple will opt for a non-traditional name, in keeping with their progressive and forward-thinking attitudes. Speculation about the name of the newest royal is already in full flow in Britain, with bookmakers listing Boris Thor and Albert as possible options.
The happy news of the expectant baby was originally announced hours after the Duke and Duchess of Sussex touched down in Sydney for their first major international tour since their wedding in May.
A Kensington Palace spokesperson said: "Their Royal Highnesses The Duke and Duchess of Sussex are very pleased to announce that The Duchess of Sussex is expecting a baby in the Spring of 2019.
"Their Royal Highnesses have appreciated all of the support they have received from people around the world since their wedding in May and are delighted to be able to share this happy news with the public."
The Queen, the Duke of Edinburgh, Prince Charles and the Duchess of Cornwall and the Duke and Duchess of Cambridge were told in person at the wedding of Princess Eugenie and Jack Brooksbank at Windsor Castle last Friday.
Rumours were rife that Meghan may be pregnant after her mother Doria Ragland was seen taking baby-care classes in Pasadena with a view to looking after her new grandchild.
The baby will supersede Prince Andrew in the line of succession, regardless of whether they are a boy or a girl, following a 2011 change in law that granted the same status to sons and daughters.
However, as the second son of the current Prince of Wales, Harry's children are not automatically given the title of prince or princess.
King George V - Harry's great great grandfather - limited titles within the royal family in 1917.
This means Harry and Meghan's first born, as a great-grandchild of the sovereign, is too far down the line of succession to be an HRH.
George V declared that: "the grandchildren of the sons of any such Sovereign in the direct male line (save only the eldest living son of the eldest son of the Prince of Wales) shall have and enjoy in all occasions the style and title enjoyed by the children of Dukes of these Our Realms."
The eldest son and heir apparent of a duke can use one of his father's lesser grade peerage titles by courtesy, according to Debrett's.
So a first son of Harry's would become Earl of Dumbarton - one of the subsidiary titles Harry received from the Queen on the morning of his wedding.
A daughter would be Lady (first name) Mountbatten-Windsor, and any subsequent sons Lord (first name) Mounbatten-Windsor.
But the Queen could make changes to allow Harry and Meghan's children to be HRHs and princes and princesses.
Ahead of Prince George's birth, the monarch issued a Letters Patent to ensure the Cambridge children had fitting titles.
Without this Princess Charlotte and Prince Louis would have been a Lady and a Lord instead, but Prince George, as the eldest son of the eldest son of the Prince of Wales, would still have been a prince.
The Queen could decide to do the same for Harry and Meghan's baby.
The Earl and Countess of Wessex's children Lady Louise Mountbatten-Windsor and Viscount Severn are actually entitled to be a princess and prince as children of the son of the sovereign.
But the couple decided, with the Queen's agreement, that their children would use the courtesy titles as sons or daughters of an earl rather than the style prince or princess.
The Duke and Duchess of Sussex announced their engagement in November 2017, shortly after confirming their relationship.
The couple married at St George's Chapel, Windsor Castle, in May this year.
Reporting: The Washington Post/The Daily Mail