Most Americans greatly overestimate the health dangers of birth control pills but underestimate their effectiveness in preventing pregnancy, according to the American College of Obstetricians and Gynecologists (ACOG).

A Gallup Poll, commissioned by ACOG and released yesterday, found that 76 percent of women and 62 percent of men surveyed said they think taking the pill, creates "substantial health risks," and most women and men think it is more hazardous than childbearing itself.

"This is in sharp contrast to the scientific evidence that has been growing in the past several years," said Dr. Luella Klein, an Emory University professor and Atlanta physician, who heads the national doctors' group. "We need to dispel the myth that contraception is more dangerous than pregnancy. Except in older women, especially older smokers, the pill saves many more lives than it costs.

"The message of the early years of the birth control pill -- of heart attacks and strokes -- has gotten through almost too well. The message of today -- of the beneficial effects of the pill -- isn't getting through as well," said Klein at a press conference yesterday.

Her group said that among the 10 million pill users, the 500 pill-related deaths each year are mostly among women over 40 or women who smoke. It noted that if these higher-risk women did not use it, the number of deaths would drop to 70 annually or 0.7 deaths for each 100,000 pill users (a woman's risk of death from childbirth or from automobile accidents is about 10 times greater, or about 10 per 100,000).

The new Gallup results also suggest that about three-fourths of all Americans approve of teaching sex education in the schools prior to high school, with roughly half of those surveyed suggesting that sex education start in elementary school.

Only 6 percent of those polled were against sex education in the schools entirely, which suggests, said Klein, most parents "want help in giving children accurate sex education at an early enough age to prevent the often tragic circumstances of unplanned and unwanted teen-age pregnancies."

The poll also found that -- by a three to one majority -- adults surveyed want contraceptive services made available to teen-agers, with a majority in favor of notifying parents when teen-agers request such services.

Eighty percent of teen pregnancies and 55 percent of all pregnancies are unplanned, noted Klein, with about half of these 3.3 million pregnancies ending in abortion each year.

Many women, according to the poll, consistently underestimate the effectiveness of all contraceptive methods and nearly 20 percent of men and women surveyed "appear to feel," she said, "that using no birth control is no less effective than taking the pill." Studies show, however, that for every 100 couples using the pill for a year, 2 to 4 women would become pregnant, compared with 60 to 80 if no contraception is used.

"All of us understand that if we go out in the rain we get wet," said Klein. Without contraception, "if you're healthy and have sex you're going to get pregnant."

The Gallup Poll was based on telephone interviews in January 1985 of a nationally representative sample of 1,036 women and 520 men.