LONDON — First came love, then came marriage. But what if — someday — it's a girl in Kate Middleton's baby carriage?

If she's the royal couple's eldest child, new rules could push the princess to a prime place in history: the first girl to accede to the throne and beat out any younger brothers.

The Commonwealth countries agreed Friday to change centuries-old rules of succession that put male heirs on the throne ahead of any older sisters, following nations such as Sweden, Belgium, the Netherlands and Norway that have scrapped male primogeniture.

The move is a baby step — the changes must still be approved by the legislatures of the 16 nations where she is head of state before they could take effect — but is seen as a triumph over outdated, sexist practices....