Samantha Cameron may bring to mind the paraphrase that in front of every great woman is some twit of a man stealing the limelight. The claim is that it is her decisive influence that transformed her husband David's image from traditional Tory toff to modern-day liberal, contributing to the current British Conservative party lead over Labour.

His standing, especially among women voters, has been done no harm at all by the announcement last week that 'Sam Cam' is now a lady in waiting too, expecting the couple's fourth child. It's positive news, both politically and personally, for the Tory party's first family. A newborn baby in 10 Downing Street next September (10 years after Tony Blair became the first prime minister in 150 years to father a child while in office) enhances the image of the Camerons as just another traditional young family – although one not short of a few bob.

Cameron describes his 38-year-old wife as his "secret weapon" in the forthcoming election campaign, with Tory activists saying she will be "championing social action projects" on visits to local communities. Cynics may feel the pregnancy was timed to garner more approval for the Tory leader, if only to show that he's virile and that someone's having sex with him. But others may feel it will help heal the loss of the couple's severely disabled first child, Ivan, who died last February. The six-year-old suffered from cerebral palsy and epilepsy from birth, but the couple had not expected to lose him at such a young age.

They have two other children, Nancy (6) and Arthur (4), and whether or not Samantha minds her husband using their image on Christmas cards, or in magazine spreads, is unclear. But she was reportedly embarrassed over publication last weekend of "provocative" modelling photos of her taken 14 years ago. David Cameron said "we knew one day someone might remember them. We were hoping they would never appear."

The shots were taken to promote Samantha's upmarket stationery company, Smythson, based on Bond Street. She works four days a week as creative director, and also co-owns interior des­ign store Oka with her mother Annabel.

So far, so fitting for a party leader's wife, and so seemingly conservative in every sense. She is even more a genuine toff than Cameron. And wealthier. She earns considerably more than his party-leader annual salary of £130,000, and as well as her directorships, David was forced to admit that his wife "owns a field in Scunthorpe". That refers to the 3,000 acres surrounding one of the stately homes her father inherited.

She is the daughter of Sir Reginald Sheffield, Baronet, and his wife, Lady Annabel Astor. Added to her jolly hockey sticks credentials is that interviewers say she sounds uncannily like Emma Thompson. But like so many daughters of the aristocracy, she also inherited a bohemian streak that makes her a curiosity underneath the sedate Tory-wife image. She fitted in pretty well with fellow students at Bristol art college, getting a dolphin tattoo on her ankle, and hanging out with rapper Tricky.

Cameron met the young Samantha Sheffield during a family holiday in Tuscany in 1992. He was a seriously stuffy 26-year-old and wanted to marry her within the year, but she felt she was too young at 22. If her Greenpeace-supporting friends found the idea of associating with anyone belonging to the Conservative Party repulsive, she was also unimpressed by some of the high and mighty among Cameron's political circle. A story from his biography tells of one of their first holidays together in Italy, during which the late respected journalist Robin Day tried to persuade the young Samantha to go skinny dipping. "On the way back to Siena, she turned to Cameron and asked: 'Who was that dirty old man in the blue swimming trunks?'"

The couple married in 1996, settling in exclusive North Kensington. As for home life with the man Barack Obama described as a 'lightweight', Samantha says David's a "terrible channel-flicker" watching "westerns and the three Godfather films again and again and again".

Party supporters claim that her job in retail keeps her in touch with the real world. They say she has a more grounded approach to what is really important to people and what's not. But she's "terrified" of being compared to Cherie Blair. Her sister Emily may be deputy editor of Vogue, but Sam is not averse to mixing the designer garb with a bit of Topshop or Zara. After all, she wore an ordinary £65 dress from M&S to the last Conservative conference. But whatever of her attempts to be the Tories' Everywoman, it looks like the upper-class Sam Cam will be in labour next September, if only for an hour.

Hero or Villain? Samantha Cameron

High: Winning Glamour magazine's Best Accessories Designer of the Year Award 2009 for her upmarket stationary company, Smythsons

Low: When the telephone rang during one of her first quiet weekends away with Cameron, she called out from the bed: "If that's Norman Lamont, tell him to f**k off."