FOR a decade, fertility clinics have been able to test embryos for certain genetic defects, discard the bad ones and implant the good ones in the mother's womb. About 700 children have been born as a result of this process.

Those who seek such screening — called pre-implantation diagnosis — are usually carriers of genetic diseases like cystic fibrosis or muscular dystrophy. Many have seen severe illness in their families, and want to protect their children from suffering.

But a report last week about one woman's effort to have a healthy child has provoked sharp questions about how far embryo screening should go, who should have children and who should decide. The woman carried a rare genetic mutation that made her almost certain to develop Alzheimer's disease before age 40. Her brother and sister, who also had the gene, had shown signs of dementia in their mid-30's. Their father had died at 42, with memory loss and psychological problems.

The woman wanted a family, but did not want her children to inherit the Alzheimer's gene. So at age 30 she had her embryos screened at a private clinic in Chicago, the Reproductive Genetics Institute, and a year and a half ago gave birth to a daughter who is free of the mutation.

The type of testing she had cannot be done for the more common forms of Alzheimer's disease, which affect much older people.

The Chicago case was described in the Journal of the American Medical Association, which also published a commentary written jointly by a bioethicist and a genetics expert, who questioned the woman's decision to have children at all, since she would soon be unable to take care of them and they would have to suffer through her deterioration and death.

Others disagreed. "We can't promise any child that they will always have two parents to care for them," Christina Masciangelo, a genetic counselor at the Chicago clinic, said. "Accidents happen, sicknesses occur, people get divorced."

"Who's going to judge who will be a good parent or not?" Dr. Margaret Pericak- Vance, director of the Center for Human Genetics at Duke University, said. "The ultimate decision should rest with the individual, as long as it's an educated decision. I'm not saying everybody will make the right choices. But you get into a dangerous zone when you make decisions about who can or cannot reproduce."

The United States veered into that zone early in the 20th century when the government set up a Eugenics Record Office and some states passed laws that let local authorities force people to be sterilized if they were judged to be defective. Defects included insanity, epilepsy, imbecility, alcoholism, criminal tendencies, deformities and homelessness. Tens of thousands of people were forcibly sterilized (the practice continued into the 1970's), and the American laws were used as a model by Nazi Germany, where more than 350,000 people were sterilized.

While Dr. Pericak-Vance did not question the woman's right to have children, she did have ethical concerns about using genetic tests to weed out embryos carrying a gene that might do no harm for 40 years. Currently, most pre-implantation diagnosis — and most pregnancy termination after amniocentesis — is done to prevent diseases that would cause severe illness or death in childhood, not decades later. But as more genes are identified for late-onset diseases, the question will come up again and again. And many people who would not have an abortion for a late-onset condition would find embryo selection acceptable.

"Who judges what's quality of life?" Dr. Pericak-Vance asked. "If somebody lives 30 years, 40 productive years, is that something that shouldn't happen? "

For her, it is a deeply personal question. "I had a 14-year-old son who died suddenly," she said. He had a medical condition, unknown to his parents, that caused blood clots in the brain.

"Knowing he was going to die, would I have said, `No, I don't want him?' Never. For those 14 years he was the most wonderful son you could have."

But, Dr. Pericak-Vance added, "Somebody else might say no, the loss is too great to bear."