Liberal Democrat leader Charles Kennedy and his wife Sarah are expecting their first child in April.
The Kennedys at this year's party conference in Bournemouth
The Kennedys, who were married in July 2002, said they were "absolutely delighted with the news".
Charles Kennedy, 44, said: "We have been very, very touched with messages of support from family and friends."
The baby is due in time for next year's election, and he joked: "I don't think God decided these things with a view to the British electoral timetable."
He told reporters on Thursday evening: "It's
very exciting and very nerve-wracking."
"We have made no secret of the fact that we wanted a family and we are very happy to be able to make this announcement," he added.
Mr Kennedy's parents, Ian and Mary, said: "We know how much this means to both of them; it is a marvellous time."
Sarah's parents, Peter and Jill Gurling, said: "We share Sarah and Charles's great happiness over this wonderful news."
The couple, who have been together for more that six years, married in a private ceremony at Westminster two years ago.
Thirty-four-year-old Sarah, who comes from South London, is the sister of Mr Kennedy's best friend James Gurling.
The pair met while Sarah was helping with the then Liberal Democrat leader Lord Ashdown's election campaign in 1997.