Is No 10 set to hear the patter of tiny feet again?

For 150 years, British prime ministers were free to concentrate on the statesmanlike pursuit of high-powered diplomacy and the lofty responsibility of running the country, rather than the more mundane concerns of nappy changing and midnight feeds.

Yet David Cameron could be the second prime minister in a decade forced to cope with the sleepless nights brought about by the arrival of a new member of the family, following the announcement that his wife, Samantha, is expecting their fourth child in September.

The couple were said to be "completely thrilled" last night after they announced the news that Mrs Cameron, 38, was three months' pregnant. The pregnancy had been planned long in advance, a spokeswoman for Mr Cameron said. While both Gordon Brown and Nick Clegg sent their congratulations, Mr Cameron's happy news will inevitably be seen as a smart electoral move as the parties battle for the crucial family vote.

Tony Blair became the first sitting prime minister to become father to a newborn in a century and a half when his youngest son, Leo, was born in May 2000 – although the happy event had not been planned. Mr Blair's wife, Cherie, was 45 when she gave birth. Lord John Russell, who was prime minister from 1846 to 1852, had been the last to have a child in office. Mr Brown lives in No 10 with his wife, Sarah, and two sons, John, six, and Fraser, four.

To their friends, news of Mrs Cameron's pregnancy did not come as a surprise as the couple have been open about their desire for another child. Mr Cameron mentioned in an interview last year that they were trying for another baby. "I'd certainly like to," he said. "But we'll have to wait and see if the stork drops one off." Nor has the Tory leader made any attempt to hide his broodiness. During a trip to a maternity unit in West Yorkshire at the end of last year, he wistfully told reporters: "It makes me want another one."

It is a year since their oldest son, Ivan, six, who had suffered from cerebral palsy and chronic epilepsy, died. Mr Cameron described how his son's condition meant constant hospital visits. The Camerons also have a daughter, Nancy, six, and a son, Arthur, four.

Downing Street said that Gordon and Sarah Brown were "very pleased" to hear the news of Mrs Cameron's pregnancy. Nick Clegg and his wife, Miriam Gonzalez Durantez, have also sent their congratulations.

But Mr Cameron's new arrival could help him in his struggle to win female votes. Polling has shown persistently that his party has been failing in its attempts to win the support of women. Mr Cameron had signalled just days ago that he was determined to make his wife, who is the creative director at the stationers, Smythson, a "secret weapon" in his campaign.

Her first-ever television interview was aired last week as part of an ITV special on the life of the Tory leader presented by Sir Trevor McDonald. In it, Mrs Cameron revealed her husband, "Dave", was a bit messy, something he may have to work on with the added pressures of looking after a baby.

It was soon after the show that the Tories' performance in the polls, which had been falling, began to recover. Party strategists will privately be hoping that the good will felt towards the Camerons as a result of the announcement will help the party reach the 40 per cent mark in the polls, seen as critical in delivering a Tory majority at the election.

However, Mr Cameron may be thankful that the new baby will not arrive until well after the election. Mr Clegg admitted in an interview with The Independent recently that he was suffering from sleepless nights as a result of helping to look after his one-year-old son, Miguel.

The baby vote: How it could play

Andrew Hawkins, chairman of ComRes

I would expect this pregnancy to go some way to making David Cameron more appealing. On the other hand, he already beats Gordon Brown in terms of softer questions and empathetic measures. So it could turn out to be inconsequential, but it certainly is not going to do any harm.

Siobhan Freegard, founder of Netmums

I think we all have to accept that we are influenced by the overall package. Perhaps women will have a slightly altered view of the Camerons because of the baby news. Impressions are built on many factors. Even in the playground, people always swarm around pregnant mums – that bump is just difficult to resist.

John Curtice, professor of politics at Strathclyde University

I'm not sure if this makes much difference – unless it affects Cameron's performance during the campaign. That has happened in the past with Charles Kennedy, whose first son, Donald, was born during the 2005 campaign. It either a) didn't do him much good, or b) provided him with a rather convenient cover – he appeared particularly bleary-eyed at one election press conference. Leo, the Blairs' last child, was not born during a campaign.

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