Should she go ahead and have the second child she'd always planned? She knows there's a real chance she might have another autist. After our first two sons were both found to be autistic, we had genetic counselling at the Maudsley Hospital. They estimated the chances of it happening a third time as one in twenty. But this is only an estimate; the genetic component of the condition isn't fully understood. We took the risk, and I'm very glad we did, because in addition to George and Sam I now have my almost pathologically un-autistic Jake.

My caller was wondering whether she could cope with another child with such complex and exhausting needs. But I suspect that what she was most frightened of was "losing" another child to autism, of being forced to relinquish her hopes for the child she thought she had in the face of mounting evidence of abnormality.

Few parents know they have an autistic baby. I didn't; nothing was spotted by any of the health professionals who dealt with George and Sam. It is only with the benefit of hindsight and the experience of Jake that I can recognise that my first two babies were, indeed, born autistic. The condition takes a while to show its true colours. Unlike a physically obvious condition, such as Down's syndrome, autism can only be diagnosed by behavioural observation. With a young baby, you'd have to be extremely clued up to know what to observe.

This means that most parents have months, even years, of false belief about their child. The extreme physical attractiveness of many autistic infants can add to the impression that a marvellous, normal child had been somehow "lost". For some parents, the process of grieving for the child you thought you had takes years to complete. There's a nagging residual feeling that the child could snap out of it; you almost wonder, sometimes, whether they're just putting it on. Documentary-maker Saskia Baron's autistic brother Timothy is now in his 40s. Her recurring childhood fantasy was that he was some kind of spy, just acting his autism, and that one day he would unmask himself.

This fantasy was fed by moments - or hours, or even days - when Tim would suddenly seem a lot more normal. He'd always return to his autistic ways, but these flashes gave the misleading impression that there was a "real" Tim inside, that you only had to find the right key to let him out. All of us who deal with autists have experienced this. Out of the blue you get some great eye contact, some clear and meaningful speech, even some action which feels empathetic. Sam is not very verbal and is usually emotionally aloof or indifferent, but once, when George was upset he went over to his brother and patted him on the head. "Don't cry, Georgie sweetheart," he said.

Autists who've written autobiographical accounts describe occasions when the mists lift, when the neurotypical world suddenly seems to make more sense. But, like most autistic experiences, such moments are only fragments. Autists can learn some coping strategies, but their condition will never be "cured". And the rest of us must learn to stop wishing for a cure. Acceptance is all.

That natural early assumption that your baby is normal affects a lot of things. Many parents choose mainstream school for their rising-fives because they haven't shaken off the impression that a mainstream child is what they have. Inclusion in the mainstream rarely works, but it takes some of us years to understand that "special needs" means exactly what it says.