The queen has issued a royal decree, called a Letter of Patent, that removes some of the limits on the use of the title. It was announced Wednesday, the duchess' 31st birthday, but according to People magazine, the decision was made on New Year's Eve.
The change ensures "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." Under the old rules, only the son of the eldest son of the Prince of Wales was to be called "prince"; a daughter was to be addressed as "lady."
The change comes on the heels of a proposed legislation, being enacted across the Commonwealth to remove gender discrimination in the line of succession. The change would guarantee that a first-born girl could inherit the throne, whether or not she has a younger brother.
The Duchess of Cambridge is expected to give birth this spring or early summer to the couple's first child. The pregnancy was announced in early December after the duchess was admitted to King Edward VII Hospital in London for treatment of severe morning sickness. She was said at the time to be in her first trimester.
The duchess has made only one public appearance since leaving the hospital, to hand out the BBC Sports Personality of the Year prizes in a televised ceremony on Dec. 16.