Samantha Cameron is expecting a baby in September, it was announced today.
Mrs Cameron and her husband, the Conservative Party leader David Cameron, were said to be "completely thrilled" by her pregnancy.
Mr Cameron has recently indicated in interviews - including a recent TV quiz by Alan Titchmarsh at a time when he must have known of his wife's condition - that he would be happy to add another child to their family.
A Conservative Party spokesman said: "The baby is due in September and David and Samantha are completely thrilled."
The Camerons suffered tragedy in February last year when their severely disabled elder son Ivan died aged just six.
They also have a four-year-old son, Arthur Elwen and a six-year-old daughter Nancy.
Mr Cameron has admitted to being broody several times over recent months.
Answering a question on population growth from an audience member at the Woodstock Literary Festival in September, he said: "I don't believe Britain is over-populated. I don't have any plans to reduce it.
"I would quite like to add to it, personally, by quite possibly one, at some stage in the future."
Touring a maternity unit in West Yorkshire in December he met a nine-hour-old baby and told reporters: "It makes me want another one."
Mr Cameron recently disclosed that his wife was being deployed as a "secret weapon" for the general election campaign.