Profiles such as @IamRoyalBaby are going viral on the social media website right now, just moments after the news was made public.
"Hey everyone, it's me," read 'Royal Baby's first tweet.
A few minutes later, the account posted, "I can confirm I am not ginger", which was retweeted around 400 times within 20 minutes.
The royal family had earlier confirmed: "The Queen, The Duke of Edinburgh, The Prince of Wales, The Duchess of Cornwall and Prince Harry and members of both families are delighted with the news."
The Duchess is believed to be 12 weeks pregnant, and the royal couple's child will be third in line to the throne.
> Duchess of Cambridge Kate Middleton pregnant with first child
> Baby girl named 'Hashtag' after Twitter
Duchess of Cambridge - a year in the life:
Copyright: PA Images