The body of a brain-dead pregnant woman was used as an "incubator" to keep her unborn baby alive until it had a better chance of survival outside the womb, doctors in Buffalo, N.Y., have reported.

After more than a week of artificially supporting the dead mother's vital functions, the premature infant was delivered by cesarean section, a tiny girl weighing about two pounds, crying vigorously.

A precarious period of intensive care followed. But the child, an orphan, was discharged from the hospital three months later, adopted, and so far appears to be meeting "normal milestones," said Dr. William P. Dillon, an obstetrician who helped make the agonizing life-and-death decisions involved in the case.

The case, thought to be the first of its kind, is sure to stir tremendous debate over the medical, legal, ethical and even economic questions involved.

It is an example of how recent advances in medical technology have created problems far more profound than those in medicine's historical domain. It occurs because doctors now have both the ability to keep heart and lungs working through artificial means when the brain is no longer functioning and the ability to save premature infants who once would have died.

The unusual medical circumstances of the pregnancy complicated the case. The 24-year-old woman was an epileptic who had been taking medication which greatly increased the risk of birth defects. Doctors did not know whether the baby was deprived of oxygen during the prolonged seizure that brought her mother to the hospital. And no one knew for sure why the woman's brain became so inflamed it ceased functioning.

Beyond that, the woman was unmarried and her elderly mother gave the doctors permission to do what they thought best. And because the state of New York had not adopted a statute defining brain death as death, there were potential legal problems. (Maryland, Virginia and the District do have such laws.)

"It was our decision, and in the final analysis we were plunged into an area where there was no medical experience. We assumed the mother intended the fetus to survive and would have wished life support to be continued. And we assumed that if the fetus could have been asked, it would have chosen life," Dillon said in an interview.

While there was "no hope" for the mother, he said, "we considered her as an incubator who could nurture and support a human being whose existence still depended on her."

Dillon and his colleagues at the Children's Hospital of Buffalo report their experience, which took place in early 1981, in today's issue of the Journal of the American Medical Association. They also cite a 1980 Buffalo case in which life support was stopped in an unconscious pregnant woman who also had suffered irreversible brain damage, but whose unborn child was only halfway to term.

The doctors suggest that there is a point, between 24 to 27 weeks of pregnancy, at which it becomes reasonable to attempt "extraordinary" life-support of the fetus in the womb in the face of maternal brain death. They note that at their hospital, a fetus delivered at 25 weeks of gestation has only about a one-third chance of surviving, while at 27 weeks three-fourths of the premature infants may survive. The brain-dead mother's baby was delivered at 26 weeks. A full-term birth occurs at about 38 to 40 weeks.

Dillon said that an extensive search of medical literature revealed no previous cases of successful cesarean delivery of such a young fetus following maternal brain death.

He noted that in 1976 and 1977, two unsuccessful cases were reported in New York and Colorado in which attempts were made to maintain maternal life support for two days to two weeks involving infants between 16 and 20 weeks of gestation.

Post-mortem cesarean section deliveries were performed in ancient Rome, with the first success reported in 237 B.C. But cases remain rare and have generally involved immediate delivery following traumatic death or illness when the baby is near term. Dillon said that he expects that the new cases they have described will remain "rare but there will be more cases."

Though infrequent, "this forces us to deal with claims of the fetus, the role of guardians to make decisions and questions of whether a woman in this condition is really dead," says Robert M. Veatch, of the Georgetown University Center for Bio-ethics.

He predicts that there will be "a great deal of controversy" as to what should be done. "One question is whether you ought to use such extreme measures to save the fetus. That's not a scientific question, it's a question of ethics, religion and public policy."

In a JAMA editorial, he notes that if there is disagreement among family and physicians about whether treatment should continue, different complications would arise, depending on the location. In the more than 30 states that have passed "brain death" laws, the young epileptic woman "could be viewed as a newly deceased, still-respiring cadaver." But in the remaining states, she might be viewed as living but terminally ill. "In the end there may be cases where the decision to view the patient as dead or alive is critical," says Veatch.