Kathy Evans believes every life has its defining moment, and the birth of her daughter Caoimhe, a baby with Down's syndrome, is hers. 'What do you say to a mother in a bipolar state of joy and grief?' she asks those confronted by women such as herself. 'In cases of stillbirth, there is the solid indisputable evidence of a body: lifeless, but real ... But when the baby is born alive, albeit disabled, there is confusion.'

Tuesday's Child, Evans' memoir of her experience, began as an affecting and award-winning newspaper story that evolved into a series. Although her prose can be beautiful, it's her perspective that compels. Unlike those who elect to bear a child with Down's, Evans was sideswiped by biology.

'At 11 weeks, we were offered the routine screening tests for Down's syndrome,' she writes. 'We said no. I am not a religious person but over the years I've developed some sort of godless spirituality, a belief in the human condition and all its frailties ... She could have had two heads and we wouldn't have cared.' But she did care, and Tuesday's Child is the evidence: a furore of love, bedazzlement and ambivalence.

When she first encountered her third daughter, she was startled. 'Caoimhe [pronounced Keeva] ... was born within her amniotic sac, her face squashed up against the membrane like a doll wrapped in cellophane.' In many ways, this memory is emblematic of their relationship, for no matter how vociferously Evans argues for the rights of those with Down's and records her hot aesthetic appreciation of her daughter, Down's is, for her, the membrane against which Caoimhe's identity is forever squashed.

'When I look at her I see every row, every spitted heated word, every flaw in our relationship encompassed in her mal-assembled body; like a biblical scapegoat her mottled skin is the woven cloth of our failings ... [My husband] has not indulged in secret conversations with a child who turned out to be a mere illusion, just a fantasy of the mind, who vanished the moment this changeling came, as callously as a cold-hearted lover. Like Alice in Wonderland, I was conned; I believed I was carrying a baby but on closer inspection it was a pig that lay squawking in the Duchess' arms.'

Clawing at a precipice, Evans fought to retain control - over her emotions, over her daughter's future, over life itself. Like the well-meaning doctor who, when Caoimhe was a baby, told Evans that she could grow up to work in 'a garden centre', Evans impaled herself on projections.

In the second half of the book, Evans steadies herself, and the result is powerful. 'Down's syndrome strikes at the heart of society's two most valued attributes: looks and intelligence,' she notes, raising exigent questions about our idea of what it is to be human. Added to this are questions about the relationship of disability to that which we understand to be normality, questions about the automatic association of termination with diagnoses of Down's, and - importantly - questions about the role of love in the decisions we make about our children. Never sufficiently detached to conclusively argue her case, Evans nonetheless succeeds in stirring indignation at the spiritual poverty of our bioethicists.

For Evans, 'ending the life of a foetus with Down's syndrome is ethically different from abortion in general; I see it as eugenics disguised as choice, where, in an impossibly short space of time, women find themselves in the role of society's unwitting gatekeepers, deciding who is of value and who isn't.' She points out that those with Down's can paint, sculpt, act, dance, and sing, and that yes, they can also be profoundly intellectually disabled, but only in 10 to 15 per cent of cases. The vast majority have mild to moderate impairment; some even have a normal IQ. And, as medical professionals know, Down's diagnoses are not always correct; mistakes are made, and healthy babies killed.

Tuesday's Child may not be the most measured book on the subject - Martha Beck's Expecting Adam is far more comprehensive in its scope - but it's an important document, deep and lyrical and heartfelt, and makes essential reading for those interested in evaluating their spin on human worth.

Tuesday's Child is available from dymocks.com.au