The Tory leader and Witney MP's revealed to Heart he may have delayed a Take That/Robbie Williams reunion.
Camerons expecting fourth child
Conservative leader and Witney MP David Cameron and his wife Samantha are expecting another baby.
The pair are said to be "completely thrilled" by the pregnancy. In recent interviews, Mr Cameron indicated that he would be happy to add another child to their family. The baby is due in September.
The Camerons have two surviving children - Arthur Elwen, 4, and Nancy, 6. Their eldest child, Ivan, who was severely disabled, died in February 2009, aged six.
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.
The Prime Minister and Sarah Brown were "very pleased" to hear the news of Mrs Cameron's pregnancy, Downing Street said. "They will be sending a message of best wishes to them," the premier's spokesman said.
Liberal Democrat leader Nick Clegg and his wife Miriam Gonzalez Durantez sent a message of congratulations to the Camerons. Mr Clegg said: "I am delighted for them both."
If Mr Cameron is Prime Minister when the baby is born, the new arrival will be only the third child born to a serving British Premier in the last 160 years.
Leo, the Blairs' fourth child, was born on May 20, 2000, while Francis Albert Rollo Russell, the third child of Lord John Russell, was born on July 11, 1849.
Mrs Cameron arrived at the couple's home in west London on Monday night and smiled for waiting reporters and photographers. Asked how she was feeling, she said: "Very, very delighted as you can imagine."