The new royal baby - William and Kate's third child - will be born fifth in line to the throne.
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Once the prince or princess arrives, Prince Harry will shift down the line of succession to sixth place.
The Duke of York will move to seventh and Princesses Beatrice and Eugenie to eighth and ninth.
Prince George and Princess Charlotte's younger sibling will be the Queen's sixth great-grandchild, and also a great-great-great-great-great-grandchild of Queen Victoria.
It was always thought that the duke and duchess would go on to have three children.
Kate is one of three and is close to both of her siblings.
She joked in the months leading up to her due date that her husband was "in denial" about having a third.
By having more than two children, William and Kate are following in the footsteps of the Queen and Prince Philip, who went on to have four children - although there was a gap of 10 years between their second child, Anne, and third, Andrew.
As a sibling to both future king George and "spare to the heir" Charlotte, the new baby is unlikely ever to be crowned sovereign.
Prince Andrew is the Queen and Philip's third child, but when he 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.
Australian Associated Press