How the New Royal Baby Could Set a Historic Precedent

Prince William and his wife, the Duchess of Cambridge, are expecting their third child, Kensington Palace confirmed on Monday.

The child will become the fifth in line to the British throne and, if the child is a boy, will be the first in the immediate royal family to be affected by changes to the line in succession.

The Succession to the Crown Act in 2013 ended the system of male preference primogeniture— where princes take precedence over their older sister—which had been in place since the 1701 Act of Settlement. The new law stipulates that males born after Oct. 28, 2011 cannot take precedence over their older sisters in the line of succession. Previously, daughters could only inherit the crown if there were no living sons.

What this means is that Princess Charlotte, 2, will remain fourth in line to the throne regardless of whether her new sibling is a boy. However, Prince Harry will be pushed further down the line of succession by his newest nephew or niece.

The law only came into effect in 2015 because all the countries in the Commonwealth— in which the Queen is the head of state— had to pass the necessary legislation.

The rules also allowed members of the Royal Family to marry a Roman Catholic and still become king or queen. However, a Roman Catholic royal is still barred from becoming the monarch.