David Cameron has denied suggestions resistance to changes to the rules of succession within the Commonwealth could lead to Britain having a queen but other realms, such as Australia, having a king.
Parliament recently rushed through the Succession to the Crown Act 2013 - which changes the law to make sure a first-born daughter to the Duke and Duchess of Cambridge would become queen even if the couple were to later have a son.
However, not all the other 15 countries that currently have the Queen as their head of state have ratified the law even though the prime minister said today it was a "done deal".
In Australia, the state of Queensland rejected the federally imposed change and has insisted on passing its own piece of legislation.
Canada is also organised along federal lines, and resistance from some in the French speaking state of Ottawa has caused problems for the Canadian government.
Robert Hazel, the director of the Constitution Unit at the University College London, said the British government has had to put in a "lot of diplomatic effort" to make sure the Commonwealth does not end up with both a queen and a king.
"The new law provides the eldest child will succeed whether it's a boy or a girl, but the law hasn't been brought into force," he told BBC Radio 4's World at One.
"The government has said it wont be brought into force until all the other Commonwealth countries where the Queen is head of state have made the necessary changes."
He said not only were the constitutions of Australia and Canada were "notoriously difficult to change" but some of the smaller nations such as Tuvalu were not sure what to do.
"The smallest countries ... some of them lack the capacity to recognise what change is required so they have needed quite a lot of help," he said. "They are nearly there but they are not there yet."
"When this baby is born it will be the only child and the eldest child so it will be next in line of succession. It will only become an issue if the law hasn't been changed and a second child is born and if that second child is a boy."
But speaking on BBC Radio 4's Women's Hour this morning following news that the Duchess of Cambridge had gone into labour, Cameron insisted the change to the rules was a "done deal".
"Changing the succession rules is a big achievement," he said. "Everyone has agreed. They have agreed even if legislation hasn't gone through their parliament, it will retrospectively apply to this child. Its pretty much a done deal."