Major medical organizations opposed to a government plan to notify parents of teen-age girls who use prescription contraceptives charged yesterday that it would alienate girls and endanger their health rather than increase family involvement.

"Without help, pregnancy is the price many will pay," contended Dr. George Ryan, president of the American College of Obstetricians and Gynecologists. He spoke on behalf of eight organizations, including the American Medical Association and the American Nurses Association, that represent more than 500,000 health professionals.

"This makes no sense on the grounds of health," said Ryan. He argued that the proposed rule was a "smoke screen for imposing certain moral attitudes on all" by those who would like to "turn back the clock" on sexual attitudes. "The idea that we're all going to have a Robert Young, 'Father Knows Best' kind of family is just not reality."

The administration proposal would require notification of parents within 10 days after girls under 18 get prescription birth control devices at federally funded clinics. The administration has said the regulation is necessary because parents should be notified of decisions which affect the "long-term health consequences for the adolescent."

But Ryan disagreed, saying that young women faced a five times greater chance of serious disease and death from pregnancy than from the pill or IUD, which he said posed "extremely small" risks. He noted that the diaphragm, which also requires a prescription, generally has little health risk.

He added that the regulation, which would apply only to federally funded clinics, would also be discriminatory toward poor teen-agers, while those with money could continue to get confidential help from private doctors.

Ryan estimated that "well over 100,000" pregnancies each year would result among teen-agers who do not seek birth control services out of fear that their parents would find out. He suggested that half might get abortions.

Marjory Mecklenburg, the government official in charge of family planning at the Department of Health and Human Services, disagreed. She argued that "over the long run I think that there will be fewer" unplanned pregnancies if parents "are working as part of the team" rather than being "shut out and told they don't have a role."

Mecklenburg acknowledged, however, that it was possible there may be an "upswing" in teen-age pregnancies during an "adjustment period" after the regulation goes into effect.