Laurie Penny: So this is what girl power meant for Victoria Beckham

As her celebrity stock grew, Victoria herself seemed to shrink

Victoria Beckham, who has just given birth to a baby girl, is a fascinating cultural study. What the fashion designer, model and former Spice Girl lacks in charisma, she makes up for in ruthlessly efficient dedication to the cause of celebrity. Accordingly, her daughter has been named Harper Seven, after the iron principle whereby one is only truly famous once one has given a child a really stupid name.

The gossip papers confirm that the first daughter born to the reigning power couple of British celebrity is to inherit her mother's status as a global fashion icon, much as one might inherit an antique watch, or a small European principality. The baby is hours old, but before she even had a name, she had a full wardrobe of designer clothes, including a pair of custom-made shoes by Christian Louboutin. That faint humming sound you can hear is Mary Wollstonecraft spinning in her grave.

When the artist formerly known as Posh Spice exploded on to the pop scene as one fifth of the Spice Girls in 1996, feminism was out, and "Girl Power" was in: a hypomanic, unthreatening brand of mitigated female liberation that was all about high heels, short skirts and fat paycheques. To my prepubescent ears, humming along to "Wannabe" on a Walkman on the way to school, it seemed that all you really, really needed if you wanted to be free and happy were sexy shoes, a lot of money, and a gang of cut-and-paste social stereotypes for friends. As Jennifer L Pozner observed in 1998, "it's probably a fair assumption that 'zigazig-ha' is not Spice shorthand for "subvert the dominant paradigm".

Of all the Spice Girls, Posh, the worst singer but the most skilled businesswoman, was the one for whom Girl Power really seemed to pay off. As the rest of the band drifted into public meltdowns and lacklustre solo careers, she married her dream footballer, became rich and famous, produced a small army of well-dressed children, and reinvented herself as a celebrity wife and mother, intercontinental style guru and fashion designer. Her autobiography, Learning To Fly, in which she speaks of her ambition for fame at all costs, sold almost half a million copies.

It is curious, then, that despite "having it all", Victoria Beckham-née-Adams never, ever seemed to be having a good time. In the few photographs where a smile breaks across her beautiful face, it is a rictus grin of desperate acquiescence, the sort of smile women give to menacing drunks who yell at them to "cheer up" on their way home from a disappointing party.

As her celebrity stock grew, Victoria herself seemed to shrink, appearing on billboards and bus hoardings looking pinched, harassed and desperately thin. The girl who once lip-synched about Girl Power on Top of the Pops became a pin-up for every anorexic teenager trying to starve herself to perfection. The tabloids feasted on her "vulnerability", even before her husband's alleged philandering was exposed in 2004, with endless stories of eating disorders, marital strife and terminal exhaustion. One high-profile modelling campaign for Marc Jacobs showed her literally disappearing into an enormous shopping bag, just two spindly legs in spiked stilettoes.

Did we really ever mistake this pastiche of empowerment for genuine liberation? We did, and 15 years after Girl Power, the joyless figure of Victoria Beckham still haunts the gossip press like an awful warning. You can have what you really, really want, the man, the money and the beautiful shoes for you and your children, but like a princess in a Grimm's fairytale, you must be prepared to sacrifice everything, even your smile. If the best this type of empowerment can offer our daughters is a lifetime's supply of designer heels, we may want to rethink our ambitions.