One of the few virtues attributable to that 21st-century staple of parenthood, the pregnancy manual, is the utterance it has given to the female form, not only in the more dramatic gestational phases but in childbirth itself. These irksome, hectoring how-to texts are nearly always illustrated with black-and-white images that far surpass them in candour: images of women with vast stomachs and pendulous breasts, with thighs like hams; images of women squatting and bellowing, pleading and pushing; images, finally, of the dark, wet head of one body emerging from between the legs of another.
Leafing through their pages, the first-time mother-to-be will be gobsmacked: Why, she will wonder, have I not seen this before? Why, in this supposedly permissive age, do I know so much about the tits and arse of every soap star, supermodel and page-three girl in the United Kingdom, and yet so little about the female body itself? It's a sort of inverse pornography, the shock that the sight of a pregnant woman can administer, the appetite with which, pregnant yourself, you devour this visual information in the hope that, covertly, some greater knowledge will be passed on with it; something to help you penetrate the mystery of pain and birth, to help you understand how one human life can stem entire from another.
But men can be, as I have often heard it expressed, "put off" by the sight - on paper or in the flesh - of a pregnant body revealed, even, or especially, if it belongs to someone they love. And should you go so far as to allow them into the birthing chamber - well, as I am often told by women of the older, wiser generation, we never did that. Birth does, I suppose, represent something of an aesthetic challenge to the onlooker, but I find it harder to understand revulsion for the simply pregnant form. It suggests that if men cannot look at women lustfully, they cannot look at them at all. Then again, it might be that the pregnant woman doesn't look back: by asking for neither attention nor approval from the male glance, she renders herself invisible. Except to middle-aged ladies - they take her in from top to toe, their eyes filling with reminiscence.
Back in 1991, Demi Moore was reborn as a feminist icon after she allowed her pregnant body to be photographed naked for the cover of Vanity Fair. People were shocked, it seemed, although they would have had a far bigger surprise if the photo had shown Demi a couple of weeks after the birth, rather than before. You don't win any prizes just for being fat. Since then a discernable unbuttoning, both verbal and visual, has taken place on the subject, culminating rather headily in such modern masterpieces as The Real Birth Show, a recent Channel 4 series hosted by Zoe Ball, in which women were invited to the studio not only to share their experiences of pregnancy and childbirth but to relive them shoulder to shoulder with a million strangers by permitting their wretched, endearing videos of the event to be screened on national television.
Such candour rather puts Demi in the shade, and I'm sure it elicited its proper measure of public disgust, while providing for the feminist onlooker as tentative a spectacle of revolution and revenge as the advertising of sanitary protection products during prime viewing time. But however distasteful other people might find her, the pregnant woman seems less inclined to care than is usual for her sex: indeed, I would go so far as to suggest that Demi Moore may have posed naked more readily while pregnant than she would have done not. There are several reasons for this, one of which is that it is harder to feel naked when you're pregnant, even though the sight of you may be enough to make other people cover their eyes.
Nudity implies privacy, but the pregnant woman is not alone: her body is, temporarily, tenanted. She may feel clothed or shielded by her baby, or at least conversant with it, as two nudists on a beach might feel more comfortable than one. It is possible, too, for her to feel some pride in her protuberant physique, no matter how much it horrifies other people. This is no more than certain men appear to feel for their beer-bellies; small children also seem to admire their own large tummies. Yet we depend on women for a preoccupying vanity, and it is really for this reason that pregnancy translates into a challenging visual statement. Throughout history, the willingness of a woman to contain her own flesh has gone hand in hand with the more extreme forms of human civilisation. In pregnancy, like a river in flood, she bursts her bounds, wreaking a localised but none the less definitive havoc.
Art has paid discreet, almost nonchalant, service to this spectacle: it has conferred a generalised mightiness on the female form but chosen not to follow closely its traversing of the specific wilderness that is childbearing. Jan van Eyck's painting of the Arnolfini wedding is the perfect example of this vagueness - what looks to the modern eye like an image of a pregnant woman is in fact more shaded and ambiguous than that, a female body swathed in its social, biological and emotional destiny. The Renaissance nude, too, with her cauldron-like belly, is the kind of fleshly entity long since relegated, in the modern age, to the special-needs category of visual aesthetics. "Is she pregnant?" children inevitably ask, standing in front of such canvases at the National Gallery. No, you explain, in the old days they actually let women look like that all the time. It may in fact be that the pregnant woman constitutes the last remaining visible evidence of the benighted reality of the female body: she alone, of all those who do not resemble Lara Croft, is allowed to show herself.
What then to make of Ron Mueck, the Australian "reality" sculptor whose three-dimensional essays in the pregnant form - a show described, perhaps uniquely in the history of art criticism, as "obstetric" - are shortly to arrive at the National Gallery? His Pregnant Woman has been likened to Michelangelo's David; one cannot go far wrong assuming, at least, that it's big. Lucian Freud's fat ladies, though not devoid of a certain classicism, could not, I think, claim such an antecedent. (Freud himself has started painting pregnant women, among them Kate Moss, who did her bit to make the normal pregnant body as unacceptable as the normal non-pregnant one: where celebrities pass, the airbrush inevitably follows.)
I have a photograph of Mueck's Mother and Child before me: it bears a startling resemblance to the staple pregnancy-manual shot. It depicts a woman in the moment after being delivered. Apparently, Mueck used to work in special effects, which explains why, for at least 60 seconds, I thought the statue was alive, despite the unlikely presence in the labour ward of several art-gallery types toting sunglasses and cameras. She is lying on her back, craning up to look at the infant who has been placed, umbilical cord still attached, on the deflated balloon of her stomach. The weight of the whole world resides in her naked thighs and breasts; she is human and yet monumental, ordinary and yet freakish - in the photo, the art-lovers gaze on her, appalled, with expressions of terrible concern. It is as if they, who know everything, did not know this, did not know how life renewed itself, nor what it looked like, the place they themselves had once come from.
And perhaps it can't be known, cannot be captured, this transfer or transition of self. We who were born were not witnesses to our birth: like death, it is something we are forever after trying to catch sight of. As for pregnancy, well, its disclosures, though inevitable, are kept a secret until the last. Raphael's Portrait of a Pregnant Woman expresses this inscrutability - his lady, bisected by light, is half herself and half the helpless host of a mystery, at once all-powerful and utterly given over. Before her destiny of pain and love, before the great human transaction, she quietly swells and swells.
Rachel Cusk is the author of A Life's Work: on becoming a mother (Fourth Estate). Her latest novel, The Lucky Ones, will be published by Fourth Estate in April
"Ron Mueck: making sculpture at the National Gallery" is at the National Gallery, London WC2 (020 7747 2885) from 19 March until 22 June