Surgeons at the University of California at San Francisco have performed the most extensive successful operation to date on a developing fetus while it was still in its mother's womb, according to a report in the May 31 issue of the New England Journal of Medicine.

During the operation, surgeons partially pulled the 24 1/2-week-old baby boy through an incision in his mother's womb and attached him to heart and oxygen level monitors. They then cut open the fetus to rearrange his stomach and intestines, which had been crowding into the chest through a hole in his diaphragm, the muscle that separates the lungs from abdominal organs.

They then sewed a Gore-Tex patch across the hole in the diaphragm, closed up the baby's abdomen and tucked him back inside his mother to complete his development.

The mother, Elizabeth Schultz of Waterloo, Mich., carried the baby, Blake Schultz, for seven more weeks, giving birth in August 1989. Though the baby was premature and needed to be on a respirator for about a month, he is healthy nine months later.

A second, similar operation was performed last March on a 25-week-old fetus. She fared better during the operation and was able to breathe on her own. Six weeks after birth, the baby, Devona Anderson of Tacoma, Wash., is healthy.

Michael R. Harrison, the UCSF researcher who pioneered fetal surgery, has been operating on fetuses for about 12 years, practicing on more than 1,000 fetal lambs and 200 fetal monkeys. The UCSF team has performed less complicated procedures in the past, such as placing a shunt to drain excess fluid from the brain.

The current defect, called a diaphragmatic hernia, occurs in about one in 2,000 babies. About a quarter of the babies live long enough after birth to undergo emergency surgery.

Harrison has attempted to repair the diaphragm eight times since 1984, but the first six fetuses died. The approach may provide a successful alternative to surgery after delivery.

But some physicians fear that the complicated surgery poses too great a risk to the mother and fetus and have urged doctors to approach it cautiously.