But it wasn't her parents who revealed the news.
Princes Charles made the announcement about his granddaughter at a Clarence House tea party for pilots and aircrew who fought in the Second World War.
According to the Mirror, Princes Charles recalled a conversation with his son, and said: "We were talking about grandchildren, he was saying Princess Charlotte does sleep through the night and it was much easier on mum then Prince George."
The Duke and Duchess of Cambridge have been looking after their daughter at their Norfolk home.
The couple recently released pictures of the baby princess being cuddled by her big brother, Prince George.
The photos show a cheeky Prince George looking happy and content as he puts his arm around his baby sister.
The Duke of Cambridge returned to work as a pilot at the beginning of June, after taking a month off with his newborn.
The couple announced they will christen Princess Charlotte on 5 July at St Mary Magdalene Church, Sandringham.