Britain's Prince William, the Duke of Cambridge, his wife Princess Catherine, the Duchess of Cambridge, Prince George and Princess Charlotte walk at the airfield in Hamburg Finkenwerder, Germany, July 21, 2017. [Photo/Agencies]

LONDON -- Kensington Palace said Tuesday that the duke and duchess of Cambridge, William and Kate, are "delighted" to confirm they are expecting their third child in April.

The child will be born fifth in line to the throne, pushing uncle Prince Harry into sixth place.

The duchess has been suffering from hyperemesis gravidarum, a severe form of morning sickness, but made a surprise appearance at a royal engagement on Monday when she danced with Paddington Bear at Paddington Station for a charity event, watched by William and Harry.

Her pregnancy was announced early in September because she had to skip some official engagements.

As expected, William and Kate are not revealing the sex of their baby. They left the gender of their first two children a surprise.

William and Kate, both 35, are already the proud parents of four-year-old Prince George and Princess Charlotte, who turned two in May.