Royal baby will be fifth in line to throne
- Credit: Matthew Usher
From the moment he or she draws her first breath, the Royal baby will be fifth in line to the throne.
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 had a happy home life with her sister, Pippa Matthews, and brother, James Middleton, and is close to both of her siblings.
But William may need to adjust to caring for a newborn once again.
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Kate 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 the Duke of Edinburgh, who went on to have four children.
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As a sibling to both future king George and 'spare to the heir' Charlotte, the new baby 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.
The Queen issued a Letters Patent under the Great Seal of the Realm in December 2012 when Kate was just a few months pregnant with George, 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'.