"The Duke and Duchess of Cambridge are delighted to confirm they are expecting a baby in April 2018," Kensington Palace said in a statement.
The Duke and Duchess of Cambridge had been forced to make an early announcement that they were expecting another baby last month when Kate missed a series of royal engagements due to severemorningsicknessorhyperemesis gravidarum.
But the royal couple, both 35, made an official announcement of the babys due date today.
Kate has since been recovering from her severe morning sickness and was seen at a mental health charity event at Paddington station in London yesterday after a long gap.
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Kate had suffered from the same sickness when she was pregnant with Prince George, now 4, and Princess Charlotte, who turned two this year.
Her conditionmeant she had to miss out on dropping off George at the gates when he started primary school near Kensington Palace last month.
The expected child will become the fifth in line to the throne behind Prince Charles, Prince William, Prince George and Princess Charlotte.
The couplesthird child and Queen Elizabeth II's sixth great-grandchild will also push Williams brother Harry further down the succession line to sixth.
While Kate is yet to resume her full royal schedule, Kensington Palace has announced that the royal couple will be undertaking a tour of Scandinavia at the request of the UK Foreign and Commonwealth Office (FCO).
Kate will be well into her pregnancy when they arrive in Norway and Sweden early next year for a tour, the exact dates for which have not been confirmed yet.
The palace had originally planned for the couple to tour all three countries in November, but Kates morning sickness meant the trip to Norway and Sweden had to be postponed.
William was always committed to visit Finland this year, as 2017 marks the centenary of the country's independence.