Queen Elizabeth has decided if the baby Prince William and the Duchess of Cambridge are expecting is a girl, she will be a princess.
Under the current rules, the first born son of the couple would become a prince, but a daughter would not be a princess.
She would be a lady but would not be known as Her Royal Highness.
But ahead of the proposed changes to the British royal succession, the Queen has issued a Letters Patent, which removes an anomaly that has been in place since 1917.
A notice published in the London Gazette announced the decision: "The Queen has been pleased by Letters Patent under the Great Seal of the Realm dated 31 December 2012 to declare that 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 William's great great grandfather King George V, had limited titles within the Royal Family.
It declared that "the children of any Sovereign of these Realms and the children of the sons of any such Sovereign and the eldest living son of the eldest son of the Prince of Wales shall have and at all times hold and enjoy the style title or attribute of Royal Highness with their titular dignity of Prince or Princess".
The Queen brought about the change by signing the Letters Patent before January 1.