The couple's clear delight at the pregnancy -- which was revealed before the campaign started -- masked the heartbreak at the sudden death in February last year of their first child, six-year-old Ivan, who had severe disabilities.
David Cameron has admitted he nearly gave up politics after Ivan's death and the enormous strain on Samantha was clear from the handful of photographs of the couple at the time.
They have two other children, six-year-old Nancy and Arthur, four.
Samantha combines motherhood with her role as creative director for luxury good brand Smythson, and has been credited with helping to modernise the company's once stuffy and traditional designs.
She says she has "no plans" to give up her job while her husband is prime minister.
Samantha Gwendoline Sheffield was born on April 18, 1971, the oldest daughter of Sir Reginald Sheffield, a wealthy landowner, and Annabel Jones, who owned an upmarket jeweller's.
Friends say she went out of her way to hide her aristocratic roots, going on from the private Marlborough College to study fine art at Bristol Polytechnic, now known as the University of the West of England.
There she mixed with a set far from her privileged background -- she regularly played pool with rap star Tricky, a former delinquent who has admitted he and Samantha were "unlikely mates".
Along the way, she acquired a small tattoo of a dolphin on her right ankle.
Samantha was still a teenager when she was introduced to David Cameron by her best friend Clare, his younger sister.
The couple dated while she was still a student, but he gave her early warning of his political ambitions.
"He was very upfront and said: 'I want to be an MP (Member of Parliament). If you think you would hate it, you have to say so.'. It certainly wasn't my natural inclination," she has said.
Samantha was undeterred and the couple married in 1996, when David was an advisor to then Chancellor of the Exchequer, Norman Lamont.
He became an MP in 2001 and Samantha is said to have played a major role in persuading him to enter the leadership race after Michael Howard stepped down in the wake of the Conservatives' defeat to Tony Blair's Labour in 2005.
Andrew Feldman, a friend of the Camerons, says Samantha acts as a "good barometer" of the public's taste for her husband.
"She has a good sense of what is important and what's not, but also of what's important to other people.
"She may be from very grand stock but she's tremendously grounded," he told David Cameron's biographers, Francis Elliott and James Hanning.