A human fetus has for the first time been removed from the womb, given surgery, and successfully returned to the womb to be born alive, in an important surgical advance by doctors at the University of California at San Francisco.

The surgery marks the first time that doctors have been able to operate on a fetus outside the womb without causing a miscarriage. Half a day after being born at term, the baby boy died. But his death was not caused by the surgery; it was a consequence of the original urinary defect. The mother is recovering normally from the surgery, a doctor at the university hospital said.

Although the baby ultimately did not survive, the operation still represents a success. It might open a "completely new option," a way of avoiding some abortions when parents learn their unborn babies may be defective, according to doctors familiar with the previous attempts. It may eventually be possible to treat a number of different defects with such prenatal surgery.

The male fetus, suffering from a urinary tract blockage that would have killed it shortly after birth, was removed from the womb at about the 21st week of pregnancy. Still tethered by the umbilical cord, the fetus underwent surgery to bypass the blockage and was replaced in the womb.

Doctors Michael Harrison, Mitchell S. Golbus, and Roy A. Filly performed the experimental surgery some months ago after many years of similar work in animals, according to a report by Ronald Kotulak of the Chicago Tribune that was confirmed yesterday.

The California research team that performed the surgery is the same one that performed another breakthrough in fetal surgery on April 29. In that case, they had inserted a tube through the mother's abdomen and into the bladder of the fetus to correct a similar life-threatening blockage of the urinary tract. That baby, also born normally, survived.

The urinary failure of the fetus operated on outside the womb was diagnosed by a sonogram, a common office procedure in which reflected sound waves are used like a finely tuned radar system to get an image of the fetus while it is in the womb. The pictures in this case showed abnormally enlarged bladder and urinary tubes, caused by fluid backing up behind a blocked urethra.

The surgery did not unblock the urethra, but instead made another opening so the fetus might vent its urine. The surgery was successful, but the blockage had apparently already done too much damage to the lungs.

Since a large proportion of the fluid in which the fetus rests during pregnancy is made up of fetal urine, the blockage had drastically reduced the amount of amniotic fluid in the womb at a critical time in the fetus's development. Though doctors are not sure why, much fluid is necessary to bathe the lungs so that they may develop properly.

So, even though the surgery corrected the condition and the mother carried the child to term, the damage to the lungs was apparently too great and the infant died 12 hours or so after it was born.

The doctors declined to give details of the surgery, saying they had submitted a scientific article on it to the New England Journal of Medicine, and they feared that an earlier press account would prevent their findings from being published. The journal refuses to publish papers that already have appeared in the press.

During a similar procedure with animals, the fetus is asleep and so apparently is not harmed by the surgery. The amniotic fluid is extracted and kept warm, to be put back when the fetus is returned to the womb. The fetus can be kept outside the womb for up to 30 minutes.

In the past, a chief obstacle to removing fetuses for surgery is that the operation itself would trigger labor in the mother, expelling the fetus prematurely. Drugs that prevent premature labor are a significant part of the new technique.

"I became interested in this field because it got to be too frustrating dealing with newborns with devastating birth defects," the Chicago Tribune quoted Dr. Harrison as saying, " defects that you knew could be prevented if you could operate on the fetus."

Fetal surgery, both in and out of the womb, is a new and now rapidly advancing field of research. Some 3 percent of all infants are born with serious defects, so tens of thousands of infants may eventually be candidates for some form of fetal surgery.

As one doctor who works in the field said, "There are three phases in medicine: one in which we recognize the existence of a disease, the second in which we learn to diagnose it accurately, and the third in which we can offer treatment. We have known birth defects since the Stone Age. Only now are we beginning to offer treatments. It is very hopeful."

Harrison voiced one strong caution about the new procedure. "A lot of inexperienced surgeons all over the world tried heart transplants because they wanted to get their names in the papers. I think that is terribly dangerous," Harrison told the Tribune.