Why Princess Charlotte will make history the moment she welcomes a new baby sibling.

Royal baby number three is set to arrive any day this month, but there’s a reason all eyes are on Princess Charlotte in the lead up to her new sibling’s arrival.

It’s not because the world will be looking on to see how she reacts to the fact she is no longer the ‘baby’ of the family. Instead it’s because the two-year-old – who turns three at the beginning of next month – will be making history the moment the Duke and Duchess expand their family to five.

For the first time in royal history, Princess Charlotte will be the first female to retain her claim to the throne, regardless of whether her new baby sibling is a boy or a girl.

Thanks to the Succession to the Crown Act of 2013, a female royal’s claim to the throne is no longer overtaken by the birth of a younger brother. Prior to the law’s passing, male siblings automatically preceded their older sisters when it came to the line of succession.

"Succession to the Crown not to depend on gender: In determining the succession to the Crown, the gender of a person born after 28 October 2011 does not give that person, or that person’s descendants, precedence over any other person (whenever born)," the act reads.

So, regardless of whether mum and dad bring home a Baby Albert or Baby Alice (yes these are the two most likely names for the next royal baby...), Princess Charlotte will still remain fourth in line for the throne, after her grandfather Prince Charles, father Prince William and big brother Prince George.

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The only person who will be bumped down the list of succession is the soon-to-be wed Prince Harry, who will be preceded in the line to the throne by his new niece or nephew.

With 36-year-old Kate Middleton having made her last official outing more than two weeks ago before going on maternity leave, there are other hints the arrival of royal baby number three could be announced very soon: for one, the rails outside the hospital wing where Kate Middleton is due to give birth have been given a fresh lick of paint.

The railings outside The Lindo Wing at St Marys Hospital Paddington are being painted. I suppose it’s never too early to get used to the smell of fresh paint pic.twitter.com/G4kmKTds1o

— Arthur Edwards (@ArthurJEdwards) March 27, 2018

The railing outside the Lindo Wing at St. Mary’s Hospital have recently been updated, making them picture perfect for the moment the couple introduce their baby to the public.

If the same protocol is followed as her first two children, the Palace will release an official statement when Kate is in the "early stages of labour". Then, an announcement will be made when the baby is born.

When Prince George was born, the public waited 12 hours before news of his birth was confirmed, but Princess Charlotte's debut was much quicker, with only a three hour gap between the labour and birth announcements.