Princess Charlotte will make history when her new royal sibling arrives

For the first time in history, a female royal’s spot in line to the throne won’t be bumped by the arrival of a male. Thanks to the Succession to the Crown Act 2013, birth order determines who will become the next king or queen of the U.K., regardless of gender. Had it not been in place, Charlotte, 2, would have lost her spot if Kate Middleton gave birth to a boy.

The legislation was first passed while Princess Kate was pregnant with Charlotte’s brother Prince George, now 4. It affects all royals born after Oct. 28, 2011, and also removed the disqualification placed on those arising from marriage to a Roman Catholic.

Currently, Prince Charles is the heir to the throne after his mother, Queen Elizabeth II. Prince William is second, with his kids Prince George and Prince Charlotte in the third and fourth positions, respectively.

Prince Harry, who is currently fifth in line, and Prince Andrew, Princess Beatrice and Princess Eugenie will all move down a place after the arrival of the new royal baby.

Of course, the sex of their baby-to-be is not yet known. The royal couple announced they were expecting baby No. 3 in September, and Princess Kate is expected to give birth within the next few weeks.

Meanwhile, having wrapped the final official engagement of her third pregnancy, Kate is officially on maternity leave and getting ready for a whole new chapter.

Though she stepped out with William for Easter service last Sunday, the 36-year-old is mostly nesting in Apartment 1A of Kensington Palace and helping her children adjust to the idea of a baby brother or sister.

Both kids are “excited” for the impending addition, a friend told PEOPLE while noting, “Charlotte, as the youngest, may find it hard.”

Kate “feels it’s an incredible privilege to be a mum,” added Peter Fonagy, head of the Anna Freud National Centre for Children and Families.

This article originally appeared on PEOPLE.