Katy Guest: Britney, it's time to get a life. Your own, not someone else's

In the annals of images of female sexuality, it will rank alongside Anthea Turner eating that Flake. Embarrassed, giggling, her face is that of a little girl. She looks straight at the camera with an expression that says, "Ooh, get me, I'm all naked!", as if she had just been hauled out of the bath and wrapped up in a big, fluffy towel. Harper's cover shot is about as luxuriantly sexy as a tall glass of milk and a sugar-free cookie before bedtime. And, I hate to break the news, but could Britney have made a mistake about being six months pregnant? She looks more like she has had too many carbohydrates after 8pm and is suffering from a little feminine bloating and in need of a tasty yoghurt drink.

Poor Britney Spears. Since she first squeezed her adolescent bosoms into that tight school blouse, she has gone from one role-play game to another, always getting the imagery just not quite right. Over the past five years, her tributes to the iconography of womanhood have ticked all the boxes. Virgin, whore, wife and mother, all have been imitated with the grim commitment of the true pro. At first she played the Brooke Shields of the Noughties: innocent yet knowing and sexy beyond her years, she graduated from the Mickey Mouse Club with a set of fluffy pigtails and a degree in keeping her knickers on. But when she could no longer keep up the virgin schtick (well done, Justin "big mouth" Timberlake), she had to try something else.

Britney playing the tart was slightly embarrassing. It was bad enough when she was throwing her barely ex-virgin self at Colin Farrell, the Irish bad boy. When she started pursuing Madonna, desperately trying to stick a tongue in whenever she spotted a camera, it all became a little bit silly. Madonna scored top marks for swapping spit with both Britney and her arch rival Christina Aguilera, one after the other. But when Britney could no longer lay down a B-side without performing a snog with her idol, she started to look like a groupie.

After a brief false start in Vegas, then, the portrait of Britney as besotted young newlywed was an encouraging move. Dragging in the cameras à la Sharon Osbourne, she and Kevin Federline were obviously so in love that they had started to lose their minds. In one scene in their in-house documentary, she focused her hand-held camera on her knees and howled with laughter. "They look just like boobs, but they're not," she confided. "They're my knees!" Adorable.

The trouble was, Britney and Kevin made Sharon and Ozzie look like the Waltons. It's hard to be Mrs Happy when your husband celebrates your first pregnancy by getting stoned with his mates and buying a pit bull. Britney cracked. After posting a poem on her website about overcoming a manipulative lover, then removing it, she decided to come over all earth mother.

The first thing that you notice about Britney's naked pose on the cover of Harper's Bazaar is that it is exactly like Demi Moore's. The second thing is that it is not. Whereas Demi sat side-on to the viewer, gazing somewhere above the camera with a magnificent hauteur, Britney looks straight at us, startledly clutching her pale breasts. Whereas Demi presented a swollen barrage balloon of a pregnancy, Britney offers a neat little bump. Demi was a famous matriarch, with a string of gorgeous, well-adjusted daughters. Britney has been investigated - and cleared - by Children and Family Services for dropping her son on his head.

Poor Britney Spears, who sang about being "Stronger" while taking back her cheating man, has spent a lifetime putting herself in pictures made for other, genuinely strong women. It is little wonder that she doesn't fit. In her poem "Remembrance of Who I Am" she writes about breaking the patterns and finding herself as a grown-up woman on her own. If only she could do that, and soon.