Week 20

I seem to have an early memory of seeing an ultrasound test on Playschool. Through the square window: a mummy exposing her globe of a belly with the navel inside out like the North Pole and a baby glowing on a screen ...

It can't be a real memory because it's in colour (70s beige and green) and we had a black and white telly till I was eight. I think that picture, with the doctor swishing the magic wand, the woman smiling, is a bit of Jungian flotsam - a mark of how much the ultrasound, rather than, say, the christening, has entered our collective unconscious as an icon of motherhood. Certainly it is the event most people ask after, though perhaps they want to get over as quickly as possible the moment when you press yet another blurred ultrasound photo into their palm.

It all makes for an excited atmosphere in the waiting room which, in my town, is indeed 70s green and beige. It's a shared moment: women bring their partners, their children or, if they are teenagers, their mothers. Lots seem to be. I think, as I think in all NHS waiting rooms, about how old, tall, well-nourished and rich I am, and how glad that I don't have acne any more.

No one chats much because you're advised to turn up with a full bladder to raise the uterus and, this being the NHS, there's a queue. We focus on old Hello! magazines with exquisite concentration and, if we still can, we cross our legs.

The image on the ultrasound screen doesn't look like a baby. It looks like outer space, with the nurse's indicator moving randomly over galaxies, black holes, the Milky Way, settling on a constellation she declares to be a brain. Then I see it, too, clear as Orion in the sky: the outline of a head, a nose, a moving arm, and the whole thing is as teary and epiphanic as generally advertised. Baby waves: we giggle. It briefly occurs to me that the horror of finding something wrong or something missing in this much-hyped moment must be extreme.

They did give me a little black and white photo to take away. I'm very fond of it: Baby in profile to the waist (they don't tell you gender in my hospital), lying on his/her back, hands raised in benediction, very peaceful and ancient-looking. They certified that he/she was an "active singleton", which pleases me, since most of my friends are active singletons, too. They also gave me - and this I was unprepared for - a set of her measurements, together with a graph so I can chart them against the national average. My baby's girth, I learn, is in the 97th percentile of largeness, but his legs only in the 55th. She is the shape of a power lifter, in other words, of a sumo wrestler, of Ganesh the Elephant God.

Already, I am worried. I had wanted, above all things, for my baby to inherit from his father the long, fleet legs that allowed him to sprint past playground tormentors and grow up with an intact body image. Already, I am talking myself into maternal self-blame: that long abdomen and those short limbs are mine, and they're hopeless in trousers and stumpy round the track. Baby will never be able to wear hipsters and PE teachers will torture him, and it will all be my genetic fault. I expect the nurse spent ages on the brain because the self-doubt cortex was abnormally large.

And this is only Baby's first measuring. She will be given national scores at birth, at four, at 11, at 16. Never, in fact, have mothers had access to such a wide spectrum of scores: not just physical but mental, psychological, genetic. I'll know whether or not Baby is smiling, burping, crawling, drawing, relating to his peers, playing games or recognising farmyard noises at the right time.

It's not that I'll blame him, either. Instead, I'll debate endlessly whether it is my genes that are causing it, or something in the too stimulating/not stimulating environment I provide, or even the glass of champagne I drank at the millennium, since the latest survey says that embryos whose mothers drink don't respond so well to whistles in the womb.

I wish I knew none of these things. Or knew them and didn't care. I wish I could accept this first child as if he were the last of seven and love him and nourish him and be equably, vaguely surprised when he turned out to have long legs or short ones, to enjoy chemistry or be a champion sprinter. I wish I could keep looking at him like a constellation of immeasurable stars and be awed and moved each time they took on a human, recognisable form.