A shake-up of the rules of succession to the British crown has now come into force, removing male bias.
If Prince William and Kate Middleton's new baby is a girl, she will now not be overtaken by any future younger brothers.
A member of the Royal Family can now also marry a Catholic and become monarch, but a Catholic royal still cannot be king or queen.
The Act was passed by Parliament in 2013, but all the countries in which the Queen is head of state had to pass any necessary legislation before it took effect.
Deputy Prime Minister Nick Clegg confirmed the Succession to the Crown Act had come into force in a written ministerial statement.
Previously, under the ancient rules of male primogeniture, royal sons took precedence over their female siblings, even leapfrogging first-born royal daughters.
The Act also replaces the outdated Royal Marriages Act 1772 and means that only the first six in line to the throne need the Queen's consent to marry - replacing a long-standing procedure affecting many distant royal relations who are descendants of George II.
Princess Eugenie, who is seventh in line to the throne, no longer needs her grandmother’s permission to wed, while Princess Beatrice still needs the Queen’s consent – but only if she gets engaged before the arrival of William and Kate’s second child, after which Beatrice drops down to seventh in line.
Six realms in addition to the UK legislated for the changes: Australia, Barbados, Canada, New Zealand, St Kitts and Nevis and St Vincent and the Grenadines; while nine concluded that the legislation was not necessary: Antigua and Barbuda, Bahamas, Belize, Grenada, Jamaica, Papua New Guinea, St Lucia, Solomon Islands and Tuvalu.
Mr Clegg wrote: “The Act reflects this Government’s emphasis on equality by removing centuries of discrimination on both religious and gender grounds. The Act puts in place succession laws that are fit for the 21st century and for a modern constitutional monarchy.”
The changes to the rules of succession were rushed through Parliament ahead of the birth of Prince George in 2013.
But the arrival of a first born son for William and Kate meant it will not be until the next generation that the law affects who accedes to the throne.
George would have traditionally been a future monarch without the modernising legislation.
The male primogeniture principle has long been criticised as outdated and discriminatory and moves towards constitutional change gathered pace in the wake of William and Kate’s wedding in April 2011 in anticipation they would have children.
In October 2011, British Prime Minister David Cameron announced that the 16 Commonwealth countries where the Queen is head of state had agreed to give female royals the same rights of succession as their brothers.
The current generation of royals has not been affected by the changes to male bias. It is not retrospective – meaning the Princess Royal does not jump ahead of her younger brothers the Duke of York and the Earl of Wessex.
The new legislation could affect Prince George if he decided to marry a Catholic.
The Prince of Wales reportedly expressed concerns about the implications, voicing worries about what will happen if his grandchild does wed a Catholic.
It raises the prospect of a future monarch’s child – perhaps Charles’s great-grandchild – being brought up in that faith.
This could ultimately lead to a constitutional crisis with a future king or queen, who would eventually be supreme governor of the Church of England, being barred from the throne because they were a Catholic.