Louisiana's law on abortion, described as one of the most stringent in the nation, was decimated today by a federal judge who said it interfered with a woman's privacy and with a doctor's right to practice medicine.

The 86-page decision by U.S. District Court Judge Robert F. Collins declared 15 parts of the law unconstitutional, including all its restrictions on obtaining an abortion. The state will appeal.

The only provisions Collins let stand were virtually meaningless because they require physicians to inform patients of existing social services if they choose not to abort, to keep statistics on abortions and complications in those operations, and to sign reports on these matters.

Collins nullified sections that would have: required a woman to get a second doctor's opinion before an abortion, forced a doctor to tell a patient that the operation might endanger her mental health, and instituted a 24-hour waiting period between the time a woman gave her written consent and underwent an abortion.

Acting with the backing of anti-abortion groups, the Louisiana legislature enacted the law in 1978 to restrict conditions under which a woman could get an abortion. A month before the state was scheduled to take effect, an array of New Orleans area plaintiffs -- a pregnant woman who wanted an abortion, three doctors and three abortion clinics -- filed suit, and Collins kept the law from being implemented while it was being litigated.

After two months of hearing in late 1978, Collins received final briefs in January 1979.

In his decision today, the judge said that in light of the U.S. Supreme Court's historic abortion ruling in 1973, the constitutional right of privacy is "broad enough to encompass a woman's decision whether or not to terminate her pregnancy."

Critics of the state law had argued that it contained so many restrictions and "scare provisions" that it, in effect, banned abortion.

The law would have forced a doctor to tell a patient that a fetus is a life from the moment of conception and that it could survive outside the womb if the woman were more than 22 weeks pregnant.

But Collins ruled that the state cannot determine when life begins or tell a physician how to practice medicine.

The judge also threw out a provision requiring parental consent for abortions on girls under 15 years, and he invalidated a section that would have made a doctor liable for criminal prosecution.

The law's criminal provisions were "ripe for arbitrary and erratic arrests and convictions," Collins said.

The plaintiff in the case, identified only as "Margaret S." was described as being an unemployed 18-year-old mother of two. Since filing the suit, she got her abortion, said Gretchen Hollander, executive director of the Louisiana American Civil Liberties Union.

The ACLU sent out two lawyers to represent her in court.