She transformed herself effortlessly from ordinary Kate Middleton into a future queen -- but for Britain's most famous new mother, the work has just begun.

Exhausted after giving birth, 31-year-old Kate will have little time to recover before she and Prince William present their newborn son, third in line to the throne, to the world's media camped outside the hospital.

It is a public start to a very public motherhood.

Despite being the daughter of a former air hostess and the first "commoner" to marry a future king since 1660, the glamorous Duchess of Cambridge has so far taken to royal life with remarkable self-assurance.

"She has many of the attributes of a natural royal performer," said Patrick Jephson, former private secretary to William's late mother Princess Diana.

Kate's beauty and charm draw frequent comparisons with Diana -- but so does the relentless hunger with which the press follow her every move.

And while hugely popular after her fairytale wedding to Prince William in 2011, she now faces the challenge of raising a future monarch in the spotlight.

Though her family has humble roots -- Kate's great-great-grandfather was a coal miner -- her own upbringing was a privileged one.

Her parents, Michael and Carole Middleton, met as lowly British Airways employees but later made millions from a party supplies business.

Catherine, their first child, was born in the town of Reading, west of London, in 1982.

The family lived in Jordan when Kate was a toddler -- she could reportedly sing "Happy Birthday" in Arabic before English -- before returning to Britain and setting up their company Party Pieces in 1987.

Kate was sent to the expensive Marlborough College boarding school, where she mingled in the same aristocratic circles as William.

But it was not until 2001 while studying history of art at St Andrews University in Scotland that she met her husband-to-be.

Known at school for being hard-working and sporty, Kate kept up her whiter-than-white image at university.

But her halo occasionally slipped, most famously at the student fashion show where she strutted her stuff in a transparent dress.

And while there were jibes about her unaristocratic background -- William's friends allegedly whispered "doors to manual" as she passed, in reference to her mother's past as a stewardess -- they were firmly a couple by 2003.

-- 'Waity Katie' --

After university, while William began his military training, Kate worked for Party Pieces and took a part-time job as a buyer for fashion label Jigsaw.

Alone in London, she was hounded by photographers in a grim reminder of Diana, who died in a car crash in Paris in 1997, pursued by paparazzi.

Nicknamed "Waity Katie" by British tabloids, who accused her of hanging around for a proposal, it was a difficult time -- made worse when William dumped her in 2007.

Many jilted girlfriends would have got revenge by selling their story, but Kate maintained a dignified silence during what turned out to be only a brief split.

"At the time I wasn't very happy about it," she later said of the break-up. "But it made me a stronger person."

Kate's reserve has fed claims that she is boring -- one columnist described her as the "largely mute daughter of a party-hat seller".

It is rare to hear her speak -- and if she has a "naughty sense of humour", as William says, it is kept well hidden from the public.

Nonetheless, her Alexander McQueen wedding gown installed her as a fashion icon, while her mix of designer labels and high street bargains has made her a fixture on the world's "best dressed" lists.

Echoes of Diana, meanwhile, are never far away.

In September the furious royals sued over photographs of Kate sunbathing topless in France, raising fears she could face the same intrusion as William's late mother.

And as with Diana, there was constant speculation about when Kate would produce an heir.

Novelist Hilary Mantel has complained the duchess has none of the "human awkwardness and emotional incontinence" that made Diana interesting.

But the princess's former private secretary insists that there is more to Kate than meets the eye.

"She is a strong-minded woman," Patrick Jephson told AFP.

"She has her own ideas about how she should serve the title that she now holds and how she prepares herself for being queen. This is, after all, an apprenticeship for the main job."