It was the day the computer guy came to connect my modem that I found out my baby was dead. I had been tired, dreadfully tired, all weekend. By the time he came, on Monday, I could barely stand up and I had lost my voice. I tried to sit beside him as he fiddled with my password, but had to lie down. I rang the doctor, and said, without even having thought it, "I haven't felt the baby move today."

I had already rung once that day and been dealt the usual sedative platitudes fed to pregnant women - "It's perfectly normal" - but the receptionist didn't say that; she said, "Come in straight away." So I looked in on the computer guy, who was tapping away on my keyboard, and said, "I have to go out for a minute. There's something wrong with my baby."

Sitting in the waiting room of the ultrasound clinic, I put my hand on my mother's arm and said, gently, as one might to a child, "You know that the baby might be dead." I still don't know how I could have said that. My baby's dead. My baby died. I had a baby that died. The words were as unbelievable then as now.

I knew it could happen. My second cousin Naomi died of pre-eclampsia with her baby still in her womb. She was seven months' pregnant and she had had an ideal pregnancy until that day. I knew unnatural death, too. My first boyfriend, Blake, was killed in a car accident when he was 22. His preternatural beauty and brilliance lent a false logic to his death - we sang Vincent by Don McLean at his funeral - but I learned then, after two years of crying, that things didn't make sense.

It was when the radiologist said, "Are you going back to see Dr Grey?" that the silence weighing on me buckled, and I knew.

I rang my friend Tessa and told her my baby was dead. Tessa is a midwife. I met her 10 years ago in a hotel in Australia when I was in a band. I went to hospital the next day. Tessa came in an hour or so after I was admitted. I knew that if I kept looking at her smiling face, I would be OK. She showed me how to make the bed-head move. "Bed goes up, bed goes down," she said, quoting Homer Simpson.

Losing a baby is very different from other kinds of deaths. It is a physical mourning, a bodily grief. One carries about oneself a constant, palpable absence, like wearing an empty knapsack. It is a diminishing experience. It makes you weak, depressive and slightly agoraphobic.

And it is terribly lonely. You are ostracised from the world of pregnancy and of motherhood - pregnant women and new mothers are the first to shun you - and you fast learn that it is unwise to discuss your baby or your body with anyone. To do so is to invite them to say the wrong thing. What has happened to you is unimaginable, and so, unwittingly, people say the most grievous things.

I was so shocked the first time I walked down a street not pregnant and someone bumped into me. Pregnant women are hallowed beings for whom crowds part, cars stop, strangers smile. But just as a pregnant woman is a sacred object, so a mother of a dead child becomes an object of fear, a pariah. She is the embodiment of every pregnant woman's worst phantasms; she is a living death.

It wasn't until six weeks later that I went online. I typed in "stillborn", and found my group. It is a board where mothers of stillborn babies write letters to one another. It was the first site I found and I have never gone anywhere else since.

Over the next year, I spent countless hours weeping and laughing into the green glare of my computer screen as I read missives from Karen and Carrie and Shannon and Sharon, from Myrna, Nina, Rebekah, Cindy, Mindy, Jodi, Julie, Jan and Justina. I loved these women fiercely, wholly, and we poured the love we could not give our dead children on one another. We were all in the sorrowing no-man's-land between having had our babies and trying to conceive again, a nowhere in which time is measured by ovulatory cycles and trimesters, birth days and death days. So, while we talked obsessively about temperature charting, we also talked about our babies, our husbands (there were no single or gay mothers) and our grief. Our intimacy was forged in words, tears and endless descriptions of cervical mucus.

Months went by before I realised that what I was doing was participating in a self-help group. The grief of a mother may be inexpressible but it is also encyclopaedic, and our project was the communal articulation of our experience. We came to share one another's language, to speak a common tongue. Reading my later posts, I barely recognise myself: I lost the "I" of writing and could only write in the corporate voice that we had become. "Reading your response was like talking to myself," was a common refrain. Soon, there was no "I": we had become one person.

Well, almost. Insincerity, I discovered, is a vital component of female friendship, and the capacity of women for becoming one another - for empathy - often arrives at a loss of self. I developed irrational dislikes for certain members, and equally irrational fondness for others. I was astonished to find myself feeling pangs of envy - that Sharon did not reply as warmly to me as she did to Shannon, that everyone fussed over Jennifer as though she were a child, that Maureen all of a sudden stopped replying to my posts - and, worse, to find myself competing for the affections of the other women. As much as I wanted to be understood, I wanted to be liked. I would stare aghast at the screen in these realisations, appalled by my petty playground jealousy, my vanity. The screen may be a window, but it is also a mirror.

I loved Karen best, and I loved her straight away. She always managed to say exactly the right thing, and her voice to me was one of pure reason and pure love. I wrote for Karen and read for her responses; I had made her my ideal correspondent. But I loved Karen as much for her perfect punctuation as for her kindness and wisdom: I had fallen in love with her literacy.

Because our relationships were comprised entirely of words, I had made the mistake of confusing articulateness with goodness, literacy with eloquence. It was often the less literate writers who were able to speak the most clearly. "I am so depressed all the time I feel like a dead person inside a living body," wrote Shannon, and I was riveted by the rightness of her phrase - it hit me with all the force of a cliche realised.

Last summer, Tessa and I often met to go swimming. We sat in the sun chatting and looking at all the different shapes of women - the stubby middle-aged Russians, the rake-thin teenagers, the jelly-thighed new mothers - and, among everything else we talked about, Tessa would tell me about the deliveries she had done that week and I would tell her about what was happening with the women in my group.

One noon, as we were sitting looking out to sea, I felt two small hands on my shoulders and heard a little boy say, beseechingly, "Mummy". He had mistaken me for his mother, who also had short dark hair and a black swimming costume. I could feel his wet lips on my ear, and I didn't so much hear as feel the absolute trust and intimacy of his small voice, deep in my body. For one second, I knew what it felt like to be the mother of a living child, to live in the world of motherhood.

There is a line to be crossed and, at a certain point, the highly unnatural activity of spending all day writing to people you have never met becomes ordinary. But after spending four or five hours thinking and writing to all the day's correspondents, realising that the day had, in fact, gone, the evanescence of my friends, of our link, would strike me, and I would fend off the thought that our bond was as fragile as the lives of our children, and as random as their deaths. I knew it couldn't last, that our friendships were ephemeral and, quite literally, insubstantial. That which bound us would pass, and we would be drawn back into the cycle of life, some of us with children, some without.

I rarely post nowadays. I don't know anyone in the group any more; almost all my friends have moved on. Recently, Kate gave birth to Krystal Rose, LeeAnn to Trevor and Angie to Tanner Christian. Karen and Carrie are pregnant, Karen courtesy of IVF, Carrie of Clomid. Shannon has disappeared. I lurk occasionally, but the tone is not so thoughtful, the women seem more cavalier. It took hundreds of thousands of words to get to know my friends, and I don't seem to have the heart to invest in the new crowd. Disconcertingly, a couple of months ago, the site hub crashed, and all our archives are now irretrievable.

On the anniversary of my baby's death, I went with Tessa to where he is buried. It is such a long drive there that arriving always comes as a small shock. A dog had shat on his grave. I didn't mind; at least it was compostible. The grave is near the curb of a road and the earth in which he is buried is unnatural, full of asphalt and bitumen. Tessa had brought some white roses, which she placed over the dog shit. The day before, Tessa had asked if I wanted her to come with me to the cemetery and I had dithered, not wanting to presume on her kindness. But then I realised that she wanted to go - he was her baby, too.

The only answer to radical loss, said Rabbi Fox at our baby's consecration, is radical love. And there I think of Tessa. I stop, stilled by the thought of her grace and beauty - I can still feel her gentle midwife's hands pressing my abdomen, still see her dear face as she brought my son from me. Out of love, from friendship, Tessa made the worst day of my life the most miraculous thing.

There is a grease-stained fingerprint on the top right-hand corner of page 199 of my edition of Mozart Sonatas. It is Blake's fingerprint: he had turned the page for me while I was playing and he was eating fish and chips. I think of him sitting beside me every time I play Köchel 333, and I remember how I snapped at him for blemishing the page.

Blake has been dead for 15 years now. Apart from that fingerprint, I have an old pyjama top and a gingham shirt he loved, but they have not smelled of him for years. I have a few photographs. But I also have a small suitcase of his letters, letters to me. His mother burnt my letters to him, thinking that I didn't want them, that she would preserve our intimacy by destroying them. He died again the day she told me she'd burnt my letters; it was the end of our conversation.

My baby did write to me, a long black line down the middle of my torso. It, my linea nigra, has faded now - has been rubbed off; there are crumbly remnants in my belly button. How proud I was of this tattoo - it was his message to me, his love letter. My memorial, my love poem to my dear little baby, is thousands of words floating in cyberspace. Everlasting words, inaccessible, in links that are now lost.

· This is an edited version of a piece that appears in Gas and Air: Tales of Pregnancy, Birth and Beyond (Bloomsbury). To order a copy for £7.99 with free UK p&p, call the Guardian book service on 0870 066 7979.