Chancellor Gordon Brown's wife Sarah is pregnant again.
The Browns: Expecting baby in July
The baby is expected in July, Mr Brown's spokesman said. Mr Brown, 54, and Mrs Brown, 42, were said to be "absolutely delighted by the news".
The couple's first child, Jennifer Jane, who Mr Brown has said was an "inspiration", died aged 10 days after being born prematurely in 2002.
Their second child, John, was born in October 2003. Since then Mr Brown has said he is a "father first of all".
Mr Brown has said he could not listen to music for nearly a year after Jennifer Jane died, but he and Sarah had taken comfort from the thousands of letters they received from other couples who had suffered similar experiences.
Downing Street babies
After their son John, named after the politician's father, was born in October 2003, weighing 8lb 1oz, the chancellor pronounced himself part of the "happiest family in Britain".
Mr Brown, 54, has subsequently told of how much he enjoys being a father - "there are sleepless nights but there is a spring in my step every morning".
Downing Street babies were once a rarity - Tony and Cherie Blair's son Leo was the first child born to a serving prime minister for 150 years.
As chancellor, Mr Brown is based at 11 Downing Street but is widely expected to succeed next door neighbour Mr Blair as prime minister when he steps down before the next election.
The man he is expected to face at that election, 39-year-old Tory leader David Cameron, is expecting his third child to be born within the next month.
In a recent radio interview Mr Brown was asked if he would look old in comparison to the new Tory leader. He replied: "I'm a father of a two-year-old child - I feel pretty young."
The most recent high profile political birth was the arrival of Donald James during last year's election campaign to the then Liberal Democrat leader Charles Kennedy.