An airplane towing an advertisement for a bookmakers flies over the Lindo Wing of St Mary's Hospital after Britain's Catherine, the Duchess of Cambridge, was admitted after going into labour ahead of the birth of her third child, in London, April 23, 2018.

A person impersonating Britain's Queen Elizabeth is driven past the Lindo Wing of St Mary's Hospital after Britain's Catherine, the Duchess of Cambridge, gave birth to a son, in London, April 23, 2018.

Britain's Prince William arrives at the Lindo Wing of St Mary's Hospital with his children Prince George and Princess Charlotte after his wife Catherine, the Duchess of Cambridge, gave birth to a son, in London, April 23, 2018.

Britain's Catherine, the Duchess of Cambridge and Prince William leave the Lindo Wing of St Mary's Hospital with their new baby boy in London, April 23, 2018.

Britain's Catherine, the Duchess of Cambridge and Prince William leave the Lindo Wing of St Mary's Hospital with their new baby boy in London, April 23, 2018.

A supporter of the royal family holds a doll wearing a crown as he sits outside the Lindo Wing of St Mary's Hospital after Britain's Catherine, the Duchess of Cambridge, was admitted after going into labour ahead of the birth of her third child, in London, April 23, 2018.

Britain's Prince William arrives at the Lindo Wing of St Mary's Hospital with his children Prince George and Princess Charlotte after his wife Catherine, the Duchess of Cambridge, gave birth to a son, in London, April 23, 2018.

Britain's Catherine, the Duchess of Cambridge and Prince William leave the Lindo Wing of St Mary's Hospital with their new baby boy in London, April 23, 2018.

Supporters of the royal family celebrate outside the Lindo Wing of St Mary's Hospital after Britain's Catherine, the Duchess of Cambridge, gave birth to a son, in London, April 23, 2018.

Britain's Prince William leaves the Lindo Wing of St Mary's Hospital after his wife Catherine, the Duchess of Cambridge, gave birth to a son, in London, April 23, 2018.

Britain's Catherine, the Duchess of Cambridge and Prince William leave the Lindo Wing of St Mary's Hospital with their new baby boy in London, April 23, 2018.

Supporters of the royal family stand outside the Lindo Wing of St Mary's Hospital after Britain's Catherine, the Duchess of Cambridge, was admitted after going into labour ahead of the birth of her third child, in London, April 23, 2018.

Senior footman Olivia Smith and footman Heather McDonald place a notice on an easel in the forecourt of Buckingham Palace to formally announce the birth of a baby boy to the Britain's Catherine, the Duchess of Cambridge, and Prince William, in London, April 23, 2018.

Britain's Catherine, the Duchess of Cambridge and Prince William leave the Lindo Wing of St Mary's Hospital with their new baby boy in London, April 23, 2018.

Britain's Catherine, the Duchess of Cambridge and Prince William leave the Lindo Wing of St Mary's Hospital with their new baby boy in London, April 23, 2018.

Britain's Prince William gestures as he prepares to leave Lindo Wing of St Mary's Hospital with Catherine, the Duchess of Cambridge and their new baby boy in London, April 23, 2018.

Police officers watch as a couple pose for a photo while holding a baby outside the Lindo Wing of St Mary's Hospital after Britain's Catherine, the Duchess of Cambridge, was admitted after going into labour ahead of the birth of her third child, in London, April 23, 2018.

Britain's Prince William leaves the Lindo Wing of St Mary's Hospital after his wife Catherine, the Duchess of Cambridge, gave birth to a son, in London, April 23, 2018.

Britain's Catherine, the Duchess of Cambridge and Prince William leave the Lindo Wing of St Mary's Hospital with their new baby boy in London, April 23, 2018.

Britain's Catherine, the Duchess of Cambridge and Prince William leave the Lindo Wing of St Mary's Hospital with their new baby boy in London, April 23, 2018. REUTERS/Hannah Mckay

A man dressed as a town crier shouts outside the Lindo Wing of St Mary's Hospital after Britain's Catherine, the Duchess of Cambridge, gave birth to a son, in London, April 23, 2018.

Britain's Prince William arrives at the Lindo Wing of St Mary's Hospital with his children Prince George and Princess Charlotte after his wife Catherine, the Duchess of Cambridge, gave birth to a son, in London, April 23, 2018.

Britain's Catherine, the Duchess of Cambridge and Prince William leave the Lindo Wing of St Mary's Hospital with their new baby boy in London, April 23, 2018.