The couple, the Duke and Duchess of Cambridge, were “delighted” to confirm the month their child is due to be born.
The duchess has continued to suffer from the condition, but made a surprise appearance at a royal engagement on Monday.
She is due to visit Norway and Sweden with William on an official visit early in 2018.
The couple already has two children: Prince George, born in 2013, and Princess Charlotte, aged two.
Both were born at St Mary’s Hospital in London, which is the same place where Prince William was delivered in 1982.