A divided Supreme Court yesterday reentered what it calls "the sensitive and earnestly contested abortion area" by narrowing the power of a state to protect fetal life.

The court ruled in a Pennsylvania case that it is for the physician -- not a legislature or a judge -- to decide if "there is a reasonable likelihood of the fetus' sustained survival outside the womb, with or without artificial support."

The main practical impact of the decision is to deter the states from trying to forbid or regulate abortions in the period of mid-pregnancy when the fetus is approaching or in the stage of "viability."

Thus the ruling reduces the chance that physicians across the country can be sued or criminally prosecuted if they perform abortions in the fifth or sixth month of pregnancy.

In such second-trimester abortions, physicians usually favor a procedure in which a salt solution is injected into the amniotic sac. So-called saline amnio-infusion, while the method of choice for the mother's safety, is nearly always fatal to the fetus.

Bishop Thomas Kelly of the National Conference of Catholic Bishops said that the court "has abandoned its constitutionally defined role and has acted as maker of social policy, a policy which the people rightly reject."

Judith Levin, of the American Civil Liberties Union's Reproductive Freedom Project, said that the ruling "insulates good-faith medical judgment from criminal liability" while enlarging the discretion of physicians to perform abortions.

She also said that the decision appears to rule out a pending murder prosecution of Dr. Jesse J. Floyd, who was indicted in Columbia, S.C., after he performed an abortion on a woman apparently in her 25th week of pregnancy.

The abortion resulted in the birth of an enfeebled infant that survived 20 days in a hospital intensive care unit and then died.

In a 1973 abortion ruling, the court held that in the first trimester, the constitutional right of privacy protects a woman's decision to have a nontherapeutic abortion from interference by the state.

But, the court said, once the fetus becomes viable -- "potentially able to live outside the mother's womb, albeit with artificial aid" -- the state's interest in protecting fetal life empowers it to forbid such abortions.

The court noted that viability "is usually placed at about seven months (28 weeks) but may occur earlier, even at 24 weeks."

In yesterday's opinion for the court, Justice Harry A. Blackmun, who wrote the 1973 decision, said, "We thus left the point flexible for anticipated advancements in medical skill... And we have recognized no attempt to stretch the point of viability one way or another."

A year after the decision, Pennsylvania enacted a law that required a doctor to try to save the life of a fetus when his "experience, judgment or professional competence" tell him that it "is" viable, or when he has "sufficient reason to believe that the fetus may be viable."

If he determines that the fetus might survive outside the womb, he must exercise the same care to preserve its life and health as he would had the mother wanted a child. So long as a different abortion technique isn't needed to preserve the mother's life or health, he must use the method providing the best chance for the fetus to be aborted alive.

If a physician violates the law, he is "subject to such civil or criminal liability" as would pertain had the woman wanted to go to term.

In a 6-to-3 decision, the court struck down the law as impermissibly vague and the requirement for determination of viability unconstitutional on its face.

In yesterday's ruling, Blackmun cited ambiguity or "double ambiguity" in key language of the Pennsylvania law, such as the phrase "may be viable." The law doesn't clarify whether the phrase refers to the viability previously defined by the court or to "an undefined penumbral or 'gray' area prior to... viability," he said.

Despite such "confusing and ambiguous criteria," Blackmun said, the law opens a physician without fault to criminal liability even though the proof of homicide required by Pennsylvania is that an accused "intentionally, knowingly, recklessly or negligently" caused a death.

In addition, Blackmun said, "it is uncertain whether the statute permits the physician to consider his duty to the patient to be paramount... or whether it requires the physician to make a 'trade-off' between the woman's health and additional percentage points of fetal survival. Serious ethical and constitutional difficulties... lurk behind this ambiguity."

For the dissenters, Justice Byron R. White wrote that the majority, "in the name of vagueness," has issued an "edict" that "has no constitutional warrant."

The majority assertion that a physician without fault could be criminally liable is an "incredible construction" of Pennsylvania laws and ignores the specific pleading by the state to the contrary, White charged.

"I do not see how it can be seriously argued that a doctor who made a good-faith mistake about whether a fetus is or is not viable could be successfully prosecuted for criminal homicide," White said.

He was joined by Chief Justice Warren E. Burger, a member of the majority in the 1973 abortion ruling, and Justice William H. Rehnquist, who was, with White, in the 1973 minority.

The case began with a constitutional challenge to the law filed before it took effect in 1974. The main plaintiffs were Dr. John Franklin and the Obstetrical Society of Philadelphia. A panel of three federal judges ruled for them in 1977. The state appealed, but the high court upheld the panel.