A 27-year-old woman who has been declared "brain dead" is being Brooklyn hospital in the hope that the four-month-old fetus she is carrying can be saved.

The only other known effort to deliver a fetus under similar circumstances, attempted a year ago at the University of Colorado, was unsuccessful. Physicians in Denver and Washington knowledgeable about the problems involved in the Brooklyn case expressed skepticism that the fetus could be saved.

The woman, Rosemarie Maniscalco collapsed in her home on the evening of Nov. 23 and was brought by her husband to Victory Memorial Hospital in Brooklyn. Mrs. Manicalco was examined and declared "brian dead," a term that means that the brain has ceased to function and there is no chance of recovery. It remained unclear yesterday the exact nature of Mrs. Maniscalco's ailment.

Mrs. Maniscalco was placed on a respirator and her heart continued to function, according to Mildred Moriarty, administrator of Victory Memorial. According to Moriarty, the fetal heartbeat is being monitored and is thought to be "good" by physicians involved in the case.

Moriarty said the decision to put Mrs. Maniscalco on the respirator was made by physicians when she was brought in. Moriarty said the patient would not be taken off the respirator "until her heart stopped. They have two lives involved."

Under New York law, a patient is not considered to be dead until the heart stops functioning.

Mrs. Maniscalco's husband, John, could not be reached. The Associated said his wife was still alive as far as he is concerned, and "the baby is still alive."

Maniscalco, according to the Associated Press, said that his wife stood up from the dinner table screaming in pain, vomited and then passed out in his arms. At the hospital Maniscalco was told his wife was suffering from inoperable pressure on her brain.

Dr. Andre Hellegers, a professor of obstetrics and gynecology at Georgetown University and director of the Kennedy Institute for the Study of Human Reproduction and Bioethics, said that a "series of problems" would have to be overcome before the fetus could be delivered successfully.

At 16 to 20 weeks into the pregnancy, the fetus is still too small to survive outside the womb. Hellegers said the fetus would have to weigh about two pounds - a weight reached at the age of about 28 weeks - before it could be delivered, by cesarean sacrion, with an even chance of survival.

"You'd have a figthing chance at less than that, but if you want more than a 50 per cent chance, you'd have to go to 28 weeks," Hellegers said.%THellegers cited three problems complicating the situation - the amount of nutrients it is possible for a "brain dead" patient to absorb, the ability of the heart of the patient to work harder as the fetus grows and needs more nourishment,and the proportion of the increased blood flow that actually goes to the fetus. "The blood has to go to the right places carrying the right stuff," Hellegers said.

Although Mrs. Maniscalco is being fed intravenously, there is no way of knowing how well the fetal system is absorbing nutrients. Beyond that, Hellegers said, little is known about how the blood flow to the uterus is regulated, although the process is thought to be governed by hormones produced by the placenta.

Dr. Edgar Makowski, chairman of the department of obstetrics and gynecology at the University of Colorado, treated a patient under similar circumstances a year ago. Makowski, interviewed by telephone yesterday, said he convinced the patient's family to give up the effort after 48 hours.

Asked if he was skeptical that the fetus could survive. Makowski replied. "Very much so," agreeing with Hellegers analysis. "You don't know how adequate that blood supply is to that pregnant uterus. There's not way of measuring or telling." Makowski said. "I mean at 16 weeks that is almost zilch in terms of a successful outcome."

Even if the fetus did survived until it could be delivered. Makowski said, there is no way of knowing what the infant's condition would be on delivery.