Mike Skinner is a healthy bouncing baby, 2 months and 17 days old today. He's too young to know his place in medical history, and he'll never remember the surgery that probably saved his life before he was born.

Three months ago yesterday, doctors at the University of California at San Francisco operated on Mike to bypass a blockage of his urethra that could have killed him. It is an easy operation when the child is born, but Mike was still inside his mother.

He became, his doctors said, the first unborn patient to undergo surgery.

However, doctors at Boston's Peter Bent Brigham Hospital reported earlier this year that they successfully drained excess fluid fromt he brain of a potentially retarded baby.

A similar operation was also reportedly performed in a Denver hospital.

The surgery, announced yesterday, posed delicate ethical issues because the operation, although necessary to prevent serious, perhaps fatal, kidney damage, could have induced labor and likked his healthy twin sister.

After discussing the dangers and consulting with a specialist on medical ethics, the doctors and the parents, Rosa and Glen Skinner, a San Mateo airline mechanic, decided to conduct the operation but to wait until the twins were old enough so the sister could survive if birth were induced by accident.

The wait meant Mike might suffer increased kidney damage but reduced the chance that surgery would hurt Mary, his sister.

The operation may usher in a new age of prenatal operations to correct abnormalities that kill or damage the fetus if doctors wait until birth. Such operations would be primarily on the brain or lungs, doctors said. Until now, only transfusions have been performed on unborn children.

Dr. Roy A. Filly, a radiologist and expert on prenatal diagnosis, said Rosa Skinner received a routine prenatal examination because she was 41 years old, and fetal abnormalities are more common among older mothers. A sonogram, similar to an X-ray but without the damaging radiation, showed the female twin was fine but the boy had a blocked urinary tract and distended bladder. This meant urine was backing up and damaging his kidneys. Moreover, most amniotic fluid comes from urine, so he had less padding around him.

That could have led to increased pressure on him from his mother's abdomen, damaging his lungs, face and limbs. The damage to his kidneys or lungs could have been fatal, Filly said.

Fily and two colleagues had practiced similar operations on sheep and monkeys, so they tried inserting a needle through Rosa Skinner's abdomen and them through Mike's abdomen.

Using local anethetic, they tried to implant a catheter, in the case a plastic drainage tube a little more than an inch long, inside Mike's bladder to drain the urine into the sac around him. The first try worked only temporarily.

Two weeks later, on April 26, they tried again with a new catheter shaped like a J so it would stay in. Guiding the catheter into place with a needle, and using sonograms to keep track of movements Mike might make, they successfully inserted the drainage tube in the bladder.

It remained there, draining the urine, until Mike and Mary were born May 10.

The operation did not remove the blockage byt only created a bypass, so Mike will soon undergo a routine operation to permit him to urinate normally. Filly said. Damage to his kidneys and lungs was not severe and doctors predicted he will live a normal life.

Another physician on the team, pediatric surgeon Michael R. Harrington, said the same technique could be used for two other types of abnormalities. One is a blockage in the brain that prevents fluid from escaping possibly severe brain damage. The second is a hold in the diaphragm that prevents the lungs from developing properly.

Surgery to repair the diaphragm would require opening up the mother and operating on the fetus with scalpels, and operation on a "whole different order of magnitude," Harrington said. But he added he had performed the operation on sheep and monkeys and he would consider performing the operation on a fetus.

Filly and Harrington, and the third doctor, obstetrician Mitchell S. Golbus, plan to publish an account of the operation in the fall.

"The doctors were all smiling, and I was overjoyed," a happy Rosa Skinner recalled to United Press International. "I could hardly talk because I was all choked up."