The Duke and Duchess of Cambridge’s third child is due in April, Kensington Palace says.
It said the couple were “delighted”. They already have two children: Prince George, who is four, and his younger sister Charlotte, who is two.
As with her previous pregnancies, the duchess has suffered from hyperemesis gravidarum, or severe morning sickness.
The new baby will be fifth in line to the throne, after its grandfather, father and older siblings.
The duchess is expected to give birth at the private Lindo wing of St Mary’s hospital in Paddington, where her other children were born.
An official visit to Norway and Sweden, planned for November, has been delayed until early 2018 because of Catherine’s sickness.
However, Prince William is going ahead with a solo trip to Finland in November.
The duchess joined Paddington Bear for a dance on a station platform during a surprise visit to a charity event on Monday.