Once she was a supermodel, among the most fêted and aesthetically perfect creatures on the planet.

Now, at the ripe old age of 33, Cindy Crawford's star is in its descent. Yet there is still one sure way for her to be a cover girl again - get pregnant and get naked.

In celebrity speak Cindy has "done a Demi" - that is, followed in the footsteps of Hollywood actress Demi Moore, who caused a sensation when she stripped off to bare her bump for the cover of Vanity Fair.

First came Demi in 1991...

A legion of celebrities followed including Spice Girl Mel G, aka Scary Spice, who posed nude for a Sunday newspaper when she was heavily pregnant with daughter Phoenix Chi.

Whatever their professed motives - Crawford says she did it on the spur of the moment - stripping off is good for publicity, whether pregnant or not.

Just ask former BBC children's TV presenter Gail Porter, troubled supermodel Kate Moss or pop star Madonna.

On Tuesday, it emerged that one of Crawford's former supermodel cohorts, Naomi Campbell, has posed for Playboy.

The 28-year-old, who was once a darling of the high fashion world, has turned glamour model and is hotly tipped to be the magazine's "Millennial Playmate".

...then Cindy in 1999

Getting undressed for the cameras is now so par for the course that even non-celebrities are following suit. Historian Amanda Foreman raised a few eyebrows in the academic world when she went au naturel to promote her book on Georgiana, Duchess of Devonshire.

In April, 11 members of Rylstone and District Women's Institute posed for a nude calendar, while last year 35 staff at a London advertising agency - where they know a thing or two about promotion - did the same to raise their profile.

Common though it is, humans remain fascinated by each others bodies. Ever since John Lennon and Yoko Ono decided to show themselves fully for the cover of their Two Virgins album, celebrity nudity has been a headline grabber.

Psychologist Dr Mark Griffiths, of Nottingham Trent University, says publicity-seeking is just one of several reasons.

"There's a host of different reasons. Money is an obvious one - Demi Moore was supposed to have got a lot - but Gail Porter says she did it on the spur of the moment. She didn't even get paid."

A shot from the Rylstone Women's Institute calendar

The same is true of Crawford, who did her session gratis for W magazine in the US. She had been due to pose in evening gowns but did not look pregnant enough.

Deeper motives have been suggested elsewhere. Andrew Evans, an arts and media psychologist, suggests the urge to strip could be revealing an unconscious desire to have a baby.

In the case of former fresh-faced kids presenter Gail Porter, who has posed naked for both GQ - coated in baby oil - and FHM magazine, it was a bombastic attempt at changing her image.

"I'm not doing anything you wouldn't see on the beach. It's nothing really. It just makes me feel feminine," she said after the first photo shoot.

Meanwhile, Kate Winslet is said to have felt "released" after her first go at on-screen nudity.

One time Blue Peter girl Anthea Turner posed with just a python

Anthea Turner and Kate Moss might have been looking for the same experience - both stripped off while recovering from a personal crisis.

Turner said there were "many reasons" why she decided to be photographed for Tatler wearing nothing more than a live python. Her main motivation had been to raise money for the fight against breast cancer.

With Moss, it was less of a surprise. Months before the nude shoot she said in a Vogue interview: "I'll get my tits out at any chance. The more you do it, the more comfortable you are about it."

One thing which unites them all, from Jerry Hall (another pregnant poser) to Helen Mirren (who did it for the Radio Times); Madonna (see her book Sex) to Denise Van Outen (a star of the recent Tatler "nude" edition), is that they are all powerful women, in control.