The Supreme Court heard argument yesterday on the perplexing question of whether the state can compel a doctor to try to save the life of an aborted fetus.

The justices quickly found themselves in a thicket of vague legal language and the uncertainties of medical science.

Repeatedly the justices raised questions about apparent imprecision in the key words of a 1974 Pennsylvania law, later invalidated on appeal, that said when a doctor's "experience, judgment and professional competence" give him "sufficient reason to believe" that an aborted fetus "may be viable," he must try to save it.

Justice Potter Stewart said the "may" means "possible," and thus a physician who knowingly didn't try to save a fetus with a mere possibility of surviving outside the mother's womb, albeit with artificial aid, could be criminally prosecuted.

Arguing for the state, Special Assistant Attorney General Carol L. Mansmann of Pittsburgh acknowledged that "possible" is indeed a dictionary definition of "may," but said the legislature had intended "a statistical probability of survival," a much more lenient standard.

When Chief Justice Warren E. Burger reminded her that laws often fail because of the choice of words, she replied. "my concern is that we'll walk out of here and lose this case because of a poor choice of words."

But he opponent, Ronald Morris of Philadelphia, said that the "poor choice" of words makes if difficult for a woman and her doctor to know if an abortion would be criminal offense. For that reason, he said, the law is unconstitutionally vague.

Morris was low-keyed in his presentation, but Mansmann, a law teacher at Duquesne University, was impassioned. At one point, Justice Thurgood Marshall chided her for not giving him the chance to put a question.

"If I've been disrespectful, I apologize," she said.

The word "viable" also troubled the court. In the majority opinion in the Supreme Court's 1973 abortion decision, Justice Harry A. Blackmun, who was silent yesterday, didn't try to give a pinpoint definition of "viability." Instead he wrote that it "is usually placed at about seven months (28 weeks) but may occur earlier," starting with the 24th week of pregnancy.

In overturning the Pennsylvania law in September 1977, a panel of three federal judges interpreted Blackmun's opinion to mean that a state's interest in protecting fetal rights doesn't empower it to regulate abortion before the 24th week of gestation. Pennsylvania was "carving out" an impermissible third "may be viable" time period, the panel held.

This was denied by Mansmann, in an exchange with Justice Lewis S. Powell Jr.

There was no dispute that the ability of a fetus to survive can't be reliably determined simply by its age or weight. Viability is "a very fluid concept," Mansmann said. Similarly, Morris said that experts' estimate of the start of viability range from 20 to 28 weeks.

Suppose a serious fetal deformity or defect is found? In that event, the state says, its interests in protecting the child must override a desire by the mother and physician to abort. But the other side - Dr. John Franklin and the Obstetrical Society of Philadelphia - say that the mother and doctor must prevail if they decide that a viable fetus wouldn't have a "meaningful" life.

Another issue bothered several justices: If a physician aborts what may be claimed to be a viable fetus, a prosecutor could round up experts - three, say - to testify that he had erred, while the accused might have no money to hire opposing experts.

Mansmann, while recognizing that experts can be found to testify to anything, said that in this area "I think that would not happen."

After the court took the case under advisement, for probable decision by mid-1979, the argument was resumed before television cameras outside the building.

Physicians see themselves as "above the law," Mansmann said. They'd have no problem in making professional judgments about viability because they'd need only the same degree of skill that they use "all the time" in other medical areas, she said.

However, Dr. Franklin said "too many of us disagree" about whatviability means.