SAY YOU ARE the parent of an infant born with Down's syndrome plus esophageal atresia, a defect of the digestive system in which the esophagus ends in a blind pouch, unconnected to the stomach. With an operation, your baby will be able to swallow and digest; without it, your baby will die. But even if the surgery is successful, the child will still have, and will always have, Down's syndrome. Would you agree to the operation, knowing that even if the surgery is successful the baby still will be retarded?

What if your new baby had spina bifida? Would you want the doctors to operate to save the baby's life, even if they could do nothing to avoid the retardation and paralysis that occur with certain forms of spina bifida? What if your baby was born with no legs? or no lungs? or no brain? At what point do you decide, on behalf of your child, that life has become intolerable, and that death is the only merciful choice?

Straight out of any parent's nightmare, Playing God in the Nursery forces you to ask yourself such horrifying questions. And author Jeff Lyon, 42, a reporter for The Chicago Tribune, manages to keep his head about him as he tangles with the painful issues these questions raise. "It is not as difficult as it might seem to determine when the quality of life becomes unacceptable," he writes in this moving, impressive book. "Although we may not be able to describe the precise point at which the level of handicap becomes too dismal, most of us have an intuitive knowledge of where that point is."

Lyon, with a perception and sensitivity that are remarkable, marshals evidence from all sources -- from medical journals, from lawsuits, from dozens of interviews with parents, physicians, lawyers, and, most compellingly, articulate older children (and their parents) who have lived with these very handicaps. He presents his facts and figures in direct, skillful prose. He makes it clear that his intuition is usually aligned with those doctors and parents who choose to pull back on medical heroics and allow profoundly damaged babies to die.

And after all the data are in, after all the experts have been heard from, after all the heartbreaking situations have been described, Lyon in the end urges us to rely on the only moral yardstick that is truly trustworthy: "the human heart."

THE BOOK begins with an intimate account of the famed "Baby Doe" case in Bloomington, Indiana, in which parents refused intestinal surgery for their newborn son because he had Down's syndrome and other birth defects, in addition to the esophageal atresia, that might not be as easily corrected. (As it turned out, the baby's heart, which was thought to be enlarged, was found on autopsy to have been normal.) But instead of a reiteration of accounts we have read before, Lyons tells the story in graphic detail. We begin in the delivery room, where the obstetrician, Dr. Walter Owens, and the baby's parents start the emotional roller coaster "from exultation to despair" in a few fateful minutes. We move to the recovery room, where, in the presence of the pediatric consultants who are advocating treatment, Owens first suggests the option of non-treatment. "I believe there are things that are worse than having a child die," he told Lyon later, thinking of his own retarded nephew. "And one of them is that it might live."

Lyon's account of the Baby Doe case, recording in its 20 pages the life-and-death struggle fought in the courts and in the intensive care nursery during the six days of the baby's anguished life, sets up the tone of the entire book: grim, gripping, comprehensive. There are no answers in this book, no right and wrong, though Lyon makes his prejudices clear.

"It is a great mischief for physicians, who will not have to live with the long-range effects of their actions, to present a handicapped child to parents who don't want it," he writes. He shows us why, taking us into the homes of families like the Markhams, whose 2-year-old son Phillip has already gone through six operations for six major birth defects, ranging from pin-thin pulmonary arteries -- which can never be completely corrected -- to a wildly overactive pancreas that had to be removed. "It would be so much harder to lose him now," says his mother, who had asked the baby's doctor to "pull the plug" on him several times during his protracted medical ordeal -- an ordeal which might yet end in death before Phillip reaches adolescence. "When he was small and in an incubator, there was no interaction. . . . Oh, God, it's harder, now that I'm coming to know this person."

Lyon introduces us to Brian West, who had Down's syndrome and an incomplete esophagus. Brian's parents never wanted the esophagus to be closed, but they lost custody of the baby and surgery was performed anyway. In the following months, Brian suffered extreme agony because of scar tissue in the surgical esophagus; stomach juices kept backing up and dripping onto his skin through a hole in his neck, making him writhe in pain. He existed on liquids delivered straight to his stomach, had several operations in a vain attempt to remove esophageal scar tissue that interfered with his swallowing, and finally died of blood poisoning, respiratory failure, and kidney failure shortly after his second birthday. Brian's futile medical care cost more than $200,000. "Our doctor said to us recently, 'Well, I guess you guys are in a position to say, "I told you so,'''" Brian's mother told Lyon. "Maybe we are, but we're not exactly gloating."

In addition to meeting children like these, we read here about many of the landmark cases that have helped establish precedent in the emerging field of medical ethics. Lyon gives us a solid understanding, and an update, of the plights of people like Karen Ann Quinlan, the New Jersey woman whose parents were allowed to pull the plug on her in 1975 (she is still alive, and still comatose); Elizabeth Bouvia, the California woman with severe cerebral palsy who checked into a hospital for help in committing suicide (she is still alive as well); the Danville, Illinois, Siamese twins whose parents were charged with conspiracy to murder them (the babies were successfully separated in 1982, with the healthier twin getting the leg they shared); and Baby Jane Doe, the Long Island baby with spina bifida whose parents refused surgery to drain water from the brain and close up her open spine (they finally consented to brain surgery, and the baby went home with her parents at the age of 6 months facing a life of probable retardation and permanent immobility).

AND WE MEET familis whose tragedies have not made the national headlines, and who speak with awful power about the effect their damaged children have had on their lives. "She is not bringing us any joy," says one bitter mother about her daughter, a 6-year-old who spends her day in a beanbag chair, not moving, not seeing, not speaking, showing no emotion. "Maybe she has changed me as a person, but I didn't need to be changed."

Not until the end of the book do we learn that Lyon was compelled to write this book because of his brother Mike, to whom, along with their parents, the book is dedicated. At the age of 30, Mike lives in an adult group home and has the intellectual abilities of a 10-year-old. "He knows all too well how stultifying his life is and . . . it fills him with bitterness. . . . He recoils when he hears the word retarded and feels keenly the gulf that separates him from other people."

After reading about the author's relationship with Mike, we begin to understand the perspective that helped shape this book. "My brother's life has acquired momentum, and I would never suggest that he hasn't the right to complete it," Lyon offers. "But it is not the kind of existence that we would desire for our loved ones. And when we see the kind of existence that is being preserved for the most direly affected children, we find it difficult to understand how we can even speak of a 'right' to such a life. It seems more fitting to speak of a right to a dignified death."

Lyon's life with Mike, whom he visits frequently, has helped convince him that it is possible for a life to be too burdensome to live. Pro-treatment proponents, he writes, imply that "minimal lives do not have to be bad and can even be enjoyable and fulfilling." They defend the struggle to keep Baby Jane Doe alive by pointing out that she "may not be completely bedridden and may at least be vaguely aware of her surroundings." But is that enough?

To test the reliability of such assumptions, Lyon brings us back once again to the yardstick of the human heart. "The exercise of compassion," he writes, "requires seeing the congenitally impaired as they are, as human beings of flesh and blood with difficult, dreary lives. Too often, in the debate over the withholding of treatment from defective newborns, the children come to be discussed in a strangely disembodied way, as if they were subordinate to the larger issue." The power of Lyon's book is that he has successfully returned the issue of newborn intensive care from the political or ethical to the profoundly, disturbingly personal.