With their two previous children the Duke and Duchess of Cambridge chose traditional, regal names.
Prince George’s full name is George Alexander Louis.
His sister is Charlotte Elizabeth Diana, both a nod to their family trees.
What are the current favourites?
As it is not known whether they’re expecting a boy or girl, bookies have offered odds on names for both genders.
William Hill's most likely girls names are Alice (4/1), Mary (11/2), Victoria 6/1) and Elizabeth (6/1) .
And they predict the most likely boys' names are Arthur (5/1), Albert (9/1), and Jack (9/1).
What are favourite names for royal baby boys?
Albert is a name with lots of history, it being the name of the Queen’s great-great-grandfather - and father - and the beloved husband of Queen Victoria.
Richard is an older royal name, it being the name of one of England’s most famous kings, Richard the Lionheart.
Charles is the name of the first in line to the throne, so we’ll be getting a new King Charles soon and as there have been two previous kings named Charles he's likely be King Charles III.
What are the favourite names for royal baby girls?
Elizabeth is an obvious choice, it being the name of the current monarch and Charlotte’s middle name.
But seeing as it is in use and already one of the names of their children, the couple could avoid it completely.
Jane may seem like a plain name but she was one of King Henry VIII’s wives, Jane Seymour, and the name of one of the shortest reigning monarch’s, Lady Jane Grey who ruled for nine days.
Victoria is a firm favourite, after one of the most famous monarchs in British history.