What a happy birthday indeed!

Queen Elizabeth II made a groundbreaking announcement Wednesday, Jan. 9 on Kate Middleton's 31st birthday, changing a century-old decree that would finally allow Middleton's potential daughter to be called "princess."

In the original 1917 decree, made by the Queen's grandfather, King George V, daughters -- even firstborn daughters -- would be referred to as "lady," but never "princess."

Any subsequent sons would take the throne instead as "prince."

But the Queen's legal decision will give the potential daughter of the Duke and Duchess of Cambridge rights to the throne, and is a decidedly big step toward removing gender discrimination from the line of succession.

"The Queen has been pleased by Letters Patentunder 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 honor," it reads.

The Succession to the Crown Bill was published last month, and if voted into law once parliament resumes, will apply to any and all of the couple's future children as well.

The gender of the baby has not yet been announced.

This article originally appeared on Usmagazine.com: Kate Middleton, Prince William's Potential Daughter Will Be Called "Princess," Queen Elizabeth II Announces