The "bump shot", in which a pregnant woman stands proudly naked for the camera, seems to have become, these days, a vital element in any passage to motherhood.

Almost six years ago, in the weeks leading up to the birth of my first child, I bought a digital camera – it seemed to me that if you had a child you ought to have one.

The first photograph I took on it was of me, naked, in a mirror, proudly displaying my bump complete with spidery stretchmarks. What I didn't know then was that in doing this I was part of a trend.

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Few celebrities seem quite able to resist their own bump shot, and in the last week we have seen magazine shoots in which Imogen Thomas and Nell McAndrew have appeared, doing, as they say, a Demi Moore: baring their bellies as the actress did for Vanity Fair in 1991. Even ordinary pregnant ladies are being swept along by this fashion, however. There are countless sites all over the web in which women record their growing bumps. Portrait photographers who once captured bouncing babies are now offering "bump shoots". In Scotland one business offers sexy "intimate boudoir" photographs to commemorate pregnancy. There is an even an app through which blooming mums-to-be can record their growing bumps.

Anyone looking at the covers of magazines over the past decade would probably assume we were in the grip of some fertility cult in which the "bump" is our most fetishised icon. They would probably think that here was a culture not only happy and comfortable with the pregnant female form, but actively celebrating it and viewing it as sexy and desirable. They might even imagine that women in pregnancy must feel great about themselves and the physical changes they are experiencing. And they might think that the bump so often displayed was the most desired of bodily attributes, more sought after than any silicon implant.

On one level they would be right. In recent years, the bump has become, as US author and "maternity lifestyle" guru Latham Thomas, right, puts it, "the new accessory", but our relationship with the pregnant body has remained ambiguous. Alongside images of Imogen Thomas smiling and looking pleased as punch are quotes from her saying: "I've been feeling a bit low and uncomfortable about putting on weight – I didn't feel sexy any more. I've gone from 9st to 11st. I've noticed my arms have gone a bit bigger and I saw a photo of myself the other day with a double chin, but it's water retention so once the baby's out, hopefully it'll all go."

Meanwhile, the countless non-celeb websites and blogs charting bump progress often express fears and anxieties about size and shape. When American celebrity Jessica Simpson did a Demi on the front cover of Elle two years ago, she was already in talks with Weight Watchers about a deal in which she would attempt to lose her baby weight within a year.

In other words, though on a superficial level it looks as if we are revelling in the joys of pregnancy, the pressure to look sexy, flaunt that bump and give it a public life, viewed by the world, and sometimes felt or stroked by strangers, is not without its negatives. When photos are reproduced online of naked, pregnant celebrities such as Shakira, above, (who gave birth to a boy on Tuesday), often the comments range from those celebrating the images as "hot" to others expressing mild disgust. We may not be so very much happier with our pregnant bodies than we were in the days when we hid them in voluminous dresses.

This is not to say that there hasn't been a huge and mostly positive shift, but it is complicated. Like most of the images in our post-feminist era, those of pregnant women are problematic. There is a fine line between them being an expression of a very real liberation, empowerment and joy on the part of the women themselves, and simply being another objectification, another element in the pressure there is on women to look fabulous and sexy come what may.

On the positive side, there has been a movement towards women enjoying and celebrating their pregnancy rather than viewing it as a source of anxiety and fear. This has partly come out of the natural birth scene. Latham Thomas, described by some as a "rock star doula", who charges fees of up to £7500 ($12,000) for coaching women through pregnancy and birth, has written a book to convey her ideas about how pregnancy can be a time of radiance, glow and empowerment for women. Mama Glow touts itself as a book that's "suited to you – the hip, post-Sex-and-the-City siren". What's novel about it is that it takes ideas about the role of diet, exercise, mindfulness, sex and the possibility of an ecstatic birth that have been prevalent in the natural birth world, and pitches them at a hip, style-conscious, looks-aware market.

For Thomas, Mama Glow is all about "women feeling sexy and finding their power at the time of pregnancy". She adds: "I wanted to promote or bring to the surface a life that I don't think people talk about for pregnant women," she says. "Generally people don't behave as if it is cool. They might say, 'Oh you're pregnant, it's going to be troubles for the next few months, sorry to hear it.' I wanted to make it like a party and a celebration, so 'glow' is all about shining and looking radiant and feeling the very best.

It's a startling rebranding of a set of ideas that up until now have been associated with a Birkenstock-wearing image. The Mama Glow woman is urged to "put on those swanky sandals and slip into that sultry, form-fitting spaghetti-strap dress". She is encouraged to "live in the glow zone". And while doing all this, she is put on a holistic diet and regime of yoga, journal-keeping and positive thinking.

Thomas has managed to repackage many of the ideas that once seemed to belong to a strange, niche world of hippie literature as the latest must-have handbag. Indeed, her book launch took place in the showroom of a handbag designer. "Everybody," she says, "thinks that fashion and entertainment is everything and birth is like the last thing anyone wants to talk about or think about, and yet all of us are here because someone gave birth to us. I really want people to reframe the way they think about birth. So Mama Glow is all about that. It's celebrating the mother and celebrating the process."

There are in her message, she says, "a lot of principles from our past, for instance the idea of the birth mother archetype. But I'm bringing it into the current times where women are wearing high heels and Stella McCartney and looking fabulous, and aren't going to give that up because they're having a baby. You shouldn't have to sacrifice style. And you shouldn't have to sacrifice your wellbeing. But you should actually get healthier during that time, obviously, and you should look radiant. I feel like that's the missing piece."

Thomas is on to something. Pregnancy has been misrepresented. It can be a time of great energy, some of it sexual. The war stories of birth have sometimes been over-sold. The problem is that, for all the glitzy branding, her method is incredibly hard work. Though its perky girlfriend-style banter makes it sound like a breeze, and the packaging is like that of a chick-lit novel, the actual regime, if done properly (holistic, dairy-free, caffeine-free, mainly plant-based diet) represents a massive lifestyle change for most people. I'm someone who did the whole natural birth thing, and I still find it alienating. I didn't have terrible morning sickness or gestational diabetes or bloated ankles, but I still feel somehow this is a ridiculously perky take on the process. And there's a bit of me, on reading it, that can't help but feel that while it may open up, and indeed sell, a particular attitude to pregnancy to the Carries and Mirandas out there, it may also produce another layer of pressure and expectation.

What's more, Mama Glow, by focusing on pregnancy, can only make passing reference to the real problem for women. As we continue to watch the ever-expanding gallery of celebrity bumps, the burning issue is not so much how we feel about the pregnant celebrity body, but the one that haunts every owner of a burgeoning bump: how our culture looks upon the post-birth body. That, after all, is what we do not celebrate. That is what we pretend does not exist. It is what is exercised and tucked out of existence, and the looming fear of it adds extra anxiety to pregnancy. Bring on the day when someone poses on the cover of Vanity Fair with stretch marks, loose skin or a c-section scar. Bring on the post-bump shot. That would be something to celebrate.

Mama Glow: A Hip Guide to Your Fabulous Abundant Pregnancy by Latham Thomas (Hay House, £10.99) is out now