The Duke and Duchess of Cambridge are expecting their third child, the Kensington Palace said on Monday.
The Queen and both families are said to be “delighted with the news”, reports the BBC.
As with her previous two pregnancies, the Duchess, 35, is suffering from hyperemesis gravidarum or severe morning sickness.
The Duke and Duchess have one son, George (4), and one daughter, Charlotte (2).
The expected child will become the fifth in line to the throne behind Prince Charles, Prince William, Prince George and Princess Charlotte.
A change – which stops royal sons taking precedence over their female siblings in the line of succession – came into force in March 2015.
The child will be the Queen’s sixth great-grandchild. British Prime Minister Theresa May has tweeted her congratulations to the couple, calling it “fantastic news”.