Prince William and Kate Middleton are expecting their third child in April, a spokesperson has confirmed.
The Duchess of Cambridge's pregnancy was announced over a month ago, but the time scale of the pregnancy had remained a secret until now and prompted fevered speculation from fans of the royal family.
A statement from Kensington Palace said the couple were "delighted" to be welcoming the latest addition to their family.
The Duke and Duchess, who married in 2011, already have two children - Prince George, born in July 2013, and Princess Charlotte, born in May 2015.
Some have expressed hope that the baby will arrive on April 29 - which will be William and Kate's seventh wedding anniversary.
Early betting odds suggest the bookies believe the baby will be a girl, and she will be called Alice.