Royal baby unlikely ever to be crowned sovereign
The new royal baby will be born fifth in line to the throne.
As a sibling to both future king Prince George and "spare to the heir" Princess Charlotte, the Duke and Duchess of Cambridge's third child is unlikely ever to be crowned sovereign.
The Duke of York is the Queen and the Duke of Edinburgh's third child, but when Andrew was born in 1960, he leapfrogged his older sister, Princess Anne, in the line of succession.
If the Cambridges' third child is a boy, he will no longer be allowed to jump ahead of older sister Charlotte in the line of succession.
Previously, under the ancient rules of male primogeniture, royal sons took precedence over their female siblings, even leapfrogging first-born royal daughters.
But a radical shake-up of the royal succession rules removed discriminatory male bias and came into force in March 2015, affecting babies born after October 28 2011.
The Succession to the Crown Act 2013 has already affected the Duke of Gloucester's granddaughters, Senna Lewis and Lyla Gilman, whose younger brothers, born in 2012, now follow them in the line of succession.
It is rare for a third-born royal child to end up as monarch and there has been no instance so far in the House of Windsor.
But 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 his older brother, George IV, died without an heir. William IV became heir apparent at the age of 62 when his other older brother, George III's second child, Frederick, Duke of York, died.
William IV was known both as the "Sailor King" for his love of the sea and career in the Navy which began at the age of 13. He was also dubbed "Silly Billy" for his rambling speeches. He was good- natured but could also be boorish and was fond of practical jokes and swearing.
Edward VI was also a third child - Henry VIII's - and became king aged nine in 1547 until his death in 1553 at the age of 15.
William and Kate's new baby will, like Prince George and Princess Charlotte, be a great-grandchild of the Queen and a great-great-great-great-great-grandchild of Queen Victoria.
Once he or she arrives, Prince Harry will shift down the line of succession to sixth place, while the Duke of York will move to seventh and Princesses Beatrice and Eugenie to eighth and ninth.
The baby will be a prince or princess thanks to the Queen, who stepped in ahead of Prince George's birth to ensure all William's children would become HRHs with fitting titles.
The Queen issued a Letters Patent under the Great Seal of the Realm in December 2012 when Kate was just a few months pregnant, declaring "all the children of the eldest son of the Prince of Wales should have and enjoy the style, title and attribute of Royal Highness with the titular dignity of Prince or Princess prefixed to their Christian names or with such other titles of honour".
A Letters Patent in 1917, issued by George V, limited titles within the Royal Family, meaning daughters born to William or Kate would not have been an HRH but Lady (forename) Mountbatten-Windsor instead and second or later-born sons would also have lacked the HRH title and become Lord (forename) Mountbatten-Windsors rather than princes.
Here is how the line of succession will look once the new royal baby is born:
1. The Prince of Wales
2. The Duke of Cambridge
3. Prince George of Cambridge
4. Princess Charlotte of Cambridge
5. The Duke and Duchess of Cambridge's third child
6. Prince Henry of Wales (Prince Harry)
7. The Duke of York
8. Princess Beatrice of York
9. Princess Eugenie of York
10. The Earl of Wessex