Twenty years ago, heavily pregnant and struggling with the heat and heaviness of it all, a group of us would waddle into a house in south London each week for an active birth class. We were in our thirties, having first babies, marooned from careers, anxious for a range of reasons. Only Lesley seemed at ease.

Lesley McIntyre was tall, calm and stately, but with an infectious, irreverent laugh. We believed that while the rest of us would go through maternity mayhem, Lesley, instinctively, would do well.

And she did, but not in a way any of us could anticipate. After an uneventful pregnancy, Molly McIntyre was born on 19 October, three weeks after I had my daughter, who is now 19 and on a gap year in Mexico. Molly died, aged 14, on 8 February, 1999.

In the days after Molly's birth, tests failed to produce a definitive diagnosis. She was blond and cherubic, but floppy. All that was ever established was that she had a muscular abnormality. Lesley and her Norwegian husband, Marius, were told to prepare for their baby's death. 'I went to intensive care. All the other babies were tiny, and there was Molly, big and beautiful,' Lesley says. 'She opened one eye and looked at me as if to say, "Convince me it's worth making the effort to live." '

Lesley, now 53, is an award-winning photographer. During Molly's life she took hundreds of rolls of film. Fifty-five of the images have now been collected into a lyrical, touching but, above all, celebratory record, The Time of Her Life. The book is striking in its simplicity. It chronicles a short life, supremely well lived. 'We enjoyed the moment,' Lesley says. 'We didn't talk about why.'

The pictures chart how Molly's personality blossomed as she grew more frail, her body supported by splints and, ultimately, a plastic spinal corset. They show her participating with gusto in Mallorca; Italy and America; on school trips; dancing Swan Lake in a wheelchair and, her passion, dressing up. Four days before she died, Lesley took the final photograph. 'I have a brain that works and a body that doesn't,' Molly had said.

Lesley writes in her introduction: 'This book is about how to live with no guarantees. In other words, it is about what everyone deals with but pretends they don't.'

The Time of Her Life confronts one of the most powerful taboos - 16,000 children a year die before they are 18 - and challenges our reaction to the disabled.

Five years on, Lesley lives in the same 1930s mansion flat. The rooms overflow, furnished like a personal McIntyre museum: posters, photographs, newspaper clippings. In the living room is a collection of Molly's miniature perfume bottles - and a now not so new sofa. 'The last time Molly saw my father, she said, "Grandpa, mum needs a new sofa. You will see she gets one, won't you?" Lesley smiles. 'She did look out for me.'

It was a mutual support pact. Pam Kozak, a close friend, says: 'Molly absolutely trusted Lel. She made Mol feel extraordinarily safe so she was extraordinarily confident. She also had this tough inner core. I remember one cold spring day, waiting for them in the street, when Mol was four. They hadn't seen me. Lesley was carrying her and they were laughing and chatting. It was as if they were one.'

By 1996, Lesley's marriage had broken down. On her first New Year's Eve as a single parent, Molly became seriously ill. Lesley writes: 'I kept clearing her nose and throat. I realised she might die. I cried and cried. I suddenly understood that whatever life she was going to have was entirely dependent upon me. I also knew from that moment on that I lived with the inevitability of the unimaginable. My child would die before me. I gradually learnt, too, that certain key people couldn't be around a child with such fragile life expectancy. She reminded them too much that uncertainty is part of the deal of being here on Earth.'

The book has a wonderful photograph of Molly and her best friend, Daisy Mount, Pam's daughter, both 10. Daisy, now 19, is at Leeds University. 'People acted as if it was breaking a taboo, to argue with somebody in a wheelchair, but we did fight. We were normal,' Daisy explains. 'Molly looked fragile, but didn't faff about. We were always playing make-believe, being Eddy and Patsy from Ab Fab and pretending to crack open another bottle of Bolly, darling .'

In her last year Molly couldn't eat enough to keep herself alive, so was fed by tube for up to 16 hours a day, which she loathed. After weeks in hospital, Lesley decided she was confident enough with the medical paraphernalia to bring her daughter home to die in the place she loved most. At her last Christmas, Molly suggested that 'a smidge of Bolly down the tube' would be appreciated .

'I'd made the decision that I was mostly responsible for bringing Molly into the world, so I didn't see it as anyone else's responsibility to see her out,' Lesley says.

Three days before Molly died, her friends - among them Daisy, Esther and Finbar - were told the end was near. 'Those teenagers were wonderful. They visited every day, their faces ashen, but they never let on to Mol that they knew,' Pam says.

Lesley speaks highly of the NHS but she had to become a tough negotiator. She rejected the suggestion to have a steel rod inserted in Molly's spine and fought ferociously for mainstream education. And she lobbied equally determinedly for others. Repeatedly she would introduce me to mothers with special needs children and next to no support. Little has changed. England has 320,000 disabled children - only 29,000 are helped by social services.

After a long campaign, Molly was given a place at Elliott comprehensive in Putney, then one of the few London schools with a lift. Academically and socially, she did well. 'She had lots of friends,' Daisy recalls. 'She was bright and funny and sensitive to other people's feelings.'

Far from becoming bitter and angry, Lesley hasn't even asked 'Why me?' - in part, perhaps, because of her politics as a socialist and feminist. 'I've never believed we live in a just world or assumed there was fairness. Molly was such a fantastic child. For all the difficulties and grief and sense of loss, it's not an experience I would have missed.'

What helped her was an early taste of her own mortality, and her passion for photography. At 17, the car she was driving was written off by a van. 'Some workmen came to pull me out of the wreckage and the first thing I remember is one saying, "Do you fancy a gasper, love?"' Lesley smiles, her love of the surreal undimmed.

'I began to take photographs of Molly because I couldn't accept many commissions. I was bearing witness to her life. I thought long and hard about her privacy. I was determined to maintain her dignity, but I also saw this as part of the fight to persuade people to look beyond disability.'

In the first year after Molly's death, Lesley drank. 'The physical withdrawal was so painful. In the flat it was as if there were snail trails of her everywhere but no matter how hard I looked I couldn't find her. I worked as a gardener part-time for three years because that gave me a structure. Then, gradually, I began to work on the book. I don't believe in another life but I do know Molly is here in me. She's changed the way I am.'

Lesley is a volunteer, working two nights a month on the Child Death helpline 'I was like a runaway train. I wanted to talk about my child all the time. After a while you feel you can't inflict that on friends because they've heard it so many times before. But you still have this compelling need to talk.

'I have a peaceful life now. I can't think of any major decision that I made on Molly's behalf that I regret. I used to tell Mol she was a privileged child, and I really believe she was. The lives of so many children like her - whatever their abilities - are so rarely commemorated, and they deserve to be. The book is for all of them. I also hope people will find Molly beautiful, because I did.'

Every year Lesley and Daisy meet on the anniversary of Molly's death and her birthday. Last time, in Ab Fab style, they met in Harvey Nicks 'and cracked open a bottle of champers. We were gossiping and laughing so much, we forgot to give Molly a toast,' Lesley smiles. 'She would have hooted at that.'

· The Time of Her Life by Lesley McIntyre, Jonathan Cape £15, is published on 13 May. Child Death helpline, freephone 0800 282986