The Camerons' new baby, due in September, will only be the third to have been born to a serving prime minister since 1849.
Tony and Cherie Blair's son Leo won the accolade of second baby when he emerged in 2000, three years after Mr Blair swept to power.
And now David Cameron has reversed the Conservatives' fortunes, with Gordon Brown making way for him in 10 Downing Street.
Mrs Cameron has never shied from helping her husband to be open about life at their Notting Hill home and they have released smiling family portraits and video blogs to the electorate since he became party leader.
The new baby will join six-year-old Nancy and four-year-old Arthur Elwen.
The Camerons were left devastated last February when their six-year-old son Ivan died. He suffered from cerebral palsy and severe epilepsy, and his profound disability permeated the couple's lives, influencing Mr Cameron's politics.
Now in Downing Street, Mrs Cameron will continue to work as creative director of thriving high-end stationery firm Smythson.
Born in London in 1971, Samantha Sheffield was brought up on her family's sprawling estate near Scunthorpe. Her mother Annabel Jones divorced her father Sir Reginald Sheffield when Samantha was young - and subsequently married former Tory minister Lord Astor.
Samantha was a teenager when she was introduced to Mr Cameron by her friend Clare, his younger sister. The couple dated while she was an art student in Bristol and he was advising the then Chancellor Norman Lamont in London. They married in 1996.
After years of keeping a low profile, Mrs Cameron was rarely out of the newspapers during the election campaign.
Mr Cameron chose to deploy his "secret weapon" as polls predicting a hung parliament started to mount - insisting that Samantha wanted to "get out there" and do what she could.
True to her word, Mrs Cameron gave her first public interview to ITV1's Trevor McDonald, lifting the lid on life with her husband.
With echoes of Sarah Brown's touching tributes to Gordon on the Labour Party conference stage, Mrs Cameron said David was "definitely not perfect" and has "lots of very irritating habits" such as being messy around the house.
But she added: "We've been together 18 years now and we've been through some fairly tough times, and I can honestly say that I don't think in all that time he's ever let me down."
Mrs Cameron swapped her glamorous workwear for jeans and trainers on campaign visits to charities - which signalled to fashion mags how seriously she was taking her duties.
She has received ample scrutiny from fashionistas over the years. At last year's Tory party conference, she chose a simple grey polka-dot dress from Marks and Spencer and heels from Zara, showing that despite her high-powered job and aristocratic background, she was an ordinary wife and mother.
But it later emerged that the dress, costing £65 on the high street, had sold out months before and this version was made especially for her.
And there have been fashion moments Mrs Cameron would rather forget, such as the decade-old photos that emerged in the Mail on Sunday revealing her vampish side.
Modelling for a female designer friend, she donned a sheepskin dress and knee-high boots to lie on the floor cuddling a kitten, prompting tabloid headlines of "Move over Carla Bruni".
But there is no doubt that Mrs Cameron will be able to compete with the array of glamorous wives on the world stage.
COMMENT RULES: Comments that are judged to be defamatory, abusive or in bad taste are not acceptable and contributors who consistently fall below certain criteria will be permanently blacklisted. The moderator will not enter into debate with individual contributors and the moderator’s decision is final. It is Belfast Telegraph policy to close comments on court cases, tribunals and active legal investigations. We may also close comments on articles which are being targeted for abuse. Problems with commenting? email@example.com