Resolutions: There was nothing on my list about this ...

The time: 1992 The place: a london hospital

The writer Dea Birkett thought she had her whole life mapped out ahead of her. Then, out of the blue, she was told the truth about her baby daughter.

I am an inveterate list-maker. There's the Day List, which mixes mundane household tasks (buy hypoallergenic washing powder, get toilet fixed again) with work (hassle travel editor to send me somewhere hot, write to publisher about lack of reviews, chase up unpaid expenses). The first item on the Day List is always "plan day" (code for "writing a list"), so as soon as the list is written, I can cross out the top line. The Week List contains more substantial work commitments: write book review; finish chapter seven; return proofs.

The most important list of all was the Life List: earn loads of money; spend half the year abroad; buy big house in central London; become hugely successful writer by 30. When 30 passed, I drew a fine blue line through that figure and wrote "35"; shortly after my 35th birthday, I put "40". The Life List was my declaration of faith in myself. I was confident that, given time and a little bit of cheating, everything would be ticked off.

In truth, "have a baby" was never on the Life List. But when I reached my early thirties, other women of my age were having children, my boyfriend was certain that he wanted a child, and, once the decision was made, it happened that night. Already pregnant, I sneaked "have a baby" on to the list in retrospect and pretended it had been there all along. Never a matey person, I imagined motherhood would be the first club to let me in.

Being pregnant was a blessing to a listophile. There are all those things you aren't supposed to eat (pinned up in a list above the fridge in the kitchen); the endless appointments at antenatal clinic, breast-feeding and breathing-in-labour classes; the birth plan (just a glorified list); and the list of those things to take with you into hospital (bags of ice, towels, birthing stool, electric fan, sandwiches, book on how to give birth).

Child-rearing itself seemed to promise a wealth of new list-compiling possibilities, around which my day, week and life could be structured. Most tasks were so simple, such as buying pink-edged Pampers, that they could be easily deleted with a great flourish of satisfaction. My antenatal appointments were replaced by my daughter's weekly weighing sessions, 10-week check, three-month check. At the three-month check, she seemed a little floppy. I added "Buy Penelope Leach and Miriam Stoppard" to the Day List; they should be able to help with handy hints as to how to cure floppiness. List-writing and motherhood were meant for each other. Then, six months after my daughter was born, the Life List stopped.

There had been suspicions about my daughter's condition for some time. But then, who isn't worried about their first baby? Maybe, if I had been 19, I'd have put her in a frilly pink dress, tied her non-existent fuzz of hair in tiny ribbons, put a bottle into her mouth and carried on careless. Only we older mothers make such a song and dance about bringing up baby. Friends were also having concerns about their newborns - one's son wasn't putting on enough weight, another's daughter had ghastly projectile vomiting. My daughter's floppiness was just another focus for an over-anxious mother in her mid-thirties.

But soon, there were several appointments a week with specialists which other babies were not having. At yet another one, my daughter was sitting on my lap as we discussed whether she was old enough to supplement her milk with some juice, when the consultant said, in such a throwaway manner that I only just caught the the words, "Of course, your daughter has cerebral palsy." Strangely, the words I remember most are not the cold, clinical diagnosis, but the "of course". Of course? There was no "of course" about it. Cerebral palsy wasn't on the list.

Cerebral palsy is a condition which affects a child's motor abilities. If you are born with cerebral palsy, you will always have it. The degree to which a child is affected varies from so mild it passes unnoticed, to near-total impairment. It seemed that my daughter would be somewhere in the middle. The consultant added, "She will always be a disappointment to you." It is the cruellest thing that anyone has ever said to me. It is also a lie.

But from that moment, my life diverged from the way of the lists. It was as if I had been following some roughly-mapped route and, suddenly, the car swerved wildly and I began to plough through a terrain I had never intended to venture into. When a baby is diagnosed with cerebral palsy, it is unclear where in this foreign land of disability she will fall. Low expectations of children with cerebral palsy - as with all children with disabilities - inevitably encourage such children to underachieve. My daughter might do all sorts of things - and she might not. There would be no certainties. The only lists I could write were those by the day or week. When I tried to construct a Life List, it dissolved into daily tasks. I could no longer imagine what lay ahead.

We all have unwritten lists; we all make presumptions about what the future will hold. As adults, we live for an unarticulated future, some blueprint we keep secret from even ourselves. Life would be intolerable otherwise; we have to think that things will go on. Without realising it, I had had a clear assumption of how my child would turn out. Much has come true - she eats ice-cream like face-paint, chats endlessly, gossips with her girlfriends and says that, when she grows up, she wants to be a witch. But it never occurred to me that alongside these delights, she would never climb trees, never run a race, never walk to the shops. These items on my Life List, so deeply assumed that I had never written them down, were suddenly erased. I felt as if the skeleton on which the flesh of my life rested had been cut out of me.

Other people talked of our "tragedy". Friends came and grasped my hands, saying they were "so sorry", as they shook their heads from side to side, looking very concerned, slightly ridiculous and horribly sad. I looked back at them, and wondered who and what they were talking about. I did not feel like a tragic heroine, a nobly suffering parent of a child with problems. I felt like a mum. But I realised, then, that the brood of mothers I expected to be part of would never embrace me. I was different. I was touched.

Five years have passed since the diagnosis. I still search for that inclusion; I still long to belong. When I meet a parent of an able-bodied child, I know that, as she looks at me and my child, she is staring at her worst nightmare. She both pities and is afraid of us. She is so glad she is not me; she is so glad I am the afflicted one, because there can only be so many of us, and, thankfully, the devil's finger has hovered above my head.

The fear of us is so great that we are written out of the able-bodied world. Wherever I look, my child and I are not there. Reluctantly tackling Toys "R" Us this Christmas, I was the only mother who couldn't see anyone who looked just a little bit like her child grinning cheekily out from the ramparts of packaging. There are no pictures of my daughter in children's magazines, no stories with handicapped heroines, no Teletubbies with wobbly legs and wheelchairs. I now realise, after five years, that the reason I could not imagine our life was because I had never seen it before. However wild all the dreams on my Life List might have been, there was always a real-life example I had come across. I had never had to encounter disability; it had been hidden for me, too.

Now I search out visions of my daughter and myself. When, in Toys "R" Us, I spotted Mattel's latest product - Barbie's new friend Becky, splendid in her sparkly pink wheelchair - my heart momentarily soared. But on Becky's box is a charitable slogan - "Share a Smile" - the only such slogan on any toy in the store. My child is always an object of pity. And, as if the wickedness of real life must intrude even into toyland, Becky's wheelchair does not fit through the door of Barbie's dream house.

On my daughter's Christmas list was Becky. Maybe she'd get her, maybe she wouldn't. Writing something down makes it no more real than Santa Claus. However many lists you write, it can't prevent that lone thunderbolt striking.

My daughter's disability has been liberating. I now know that nothing is meant to be, no blueprint can be tattooed on to the energising anarchism of our lives, no Life List is written in stone. Other people remain certain: "I'm going to Barbados on holiday next year", "I'm not going to get pregnant for a few months", "I'll be promoted in the summer", "It'll be a wonderful wedding". But when I hear them say such things, I feel sorry for them. Bound by their life's blueprint, the childish wonder of the world is lost. Don't they know there are no certainties, only hopes?

Dea Birkett is author of Serpent in Paradise (Picador, pounds 16.99).