A prominent French scientist and his colleagues have discovered a dramatic method of regulating a woman's menstrual cycle that could lead to a monthly birth control pill instead of the daily pill now widely used.

The chemical could also be used to interrupt pregnancy within the first eight weeks, offering an alternative to surgical abortion in the early stages, its developers say.

The research was announced by Dr. Etienne-Emile Baulieu, culminating over a decade of research in his laboratory in cooperation with a French drug company. He was scheduled to present his findings to the French Academy of Sciences in Paris yesterday.

Baulieu, an internationally known expert on reproductive biochemistry, explained by telephone that the new chemical appears to act as an "anti-hormone" to block the action of progesterone, a natural hormone necessary to sustain pregnancy.

He said that it had the potential for becoming a second-generation birth control pill with fewer side effects than existing ones, but that human testing will take several years to complete.

Limited studies have been conducted on a small number of women by Dr. Walter Hermann of the University of Geneva, following extensive testing on animals.

"Biologically speaking it's definitely a revolution. Whether it will become the pill No. 2, the people have to decide," said Baulieu, who is a professor of biochemistry at the University of Paris and head of a research unit at INSERM, the French equivalent of the U. S. National Institutes of Health.

An English translation of a newly prepared paper by Baulieu and other scientists says that the new drug points to a "new methodology for menstual cycle regulation and human birth control."

U. S. scientists have not yet had a chance to review the findings in detail. Based on the information available, several experts cautioned that it will take time to see if the product lives up to its potential but were enthusiastic about the prospects.

"There is no question it's exciting," said Dr. Gabriel Bialy, head of contraceptive development at the National Institute of Child Health and Human Development. He said the French work may prove to be a "breakthrough" that could "very well represent the next generation of the birth control pill."

But, added Bialy, "as with any form of therapy, these results are preliminary . . . . How it turns out to be in the long run and how people with divergent scientific and ethical philosophies look at it, that's a horse of a different color."

"If it's what they say it is, it would really be considered a major lead, a major breakthrough if you will," said Dr. Wayne Bardin, director of the Center for Biomedical Research at the Population Council in New York.

Bardin, who recently visited Baulieu's laboratory, said that there was a "strong possibility" that his organization, which is privately funded, might help with human studies of the new drug.

Dr. Raymond Vande Wiele, chairman of obstetrics and gynecology at Columbia University, has also followed Baulieu's progress and hopes to participate in testing in this country.

Baulieu said that an international clinical trial in a number of countries is in the early discussion stage. He plans to work with the World Health Organization, which has already shown an interest.

The patent for the compound, a synthetic steroid identified only as "Ru-486," is held by Roussel-Uclaf, a French drug company.

What makes the new chemical different is the mechanism of action, which seems to interrupt the menstrual cycle in a far more specific way than existing contraceptives by blocking the action of progesterone. Bardin noted that researchers have been seeking such an agent since the late 1960s, but the French team appears to be the first to have isolated it. Progesterone is produced each month in the second half of a woman's menstrual cycle. If fertilization occurs, the levels of this hormone remain high and seem to be necessary for implantation of an embryo in the uterus and maintenance of the pregnancy.

Rather than attempt to inhibit the production of progesterone itself, the French scientists focused on blocking molecular receptors necessary for it to act. "We have made a false key that blocks the receptor," Baulieu said.

Existing birth control pills most commonly contain a combination of synthetic progesterone and estrogen hormones that are taken daily.

Baulieu envisions women taking the new pill once a month at the end of the menstrual cycle to "induce the period whether or not there is pregnancy." In three women volunteers, use of the drug for three days produced a menstrual flow that was normal but slightly more abundant than usual.

A potentially more controversial usage would be to induce abortion. It proved successful in 9 out of 11 cases of women 6 to 8 weeks pregnant, but Baulieu emphasized that further studies of the appropriate treatment dose are needed to determine what the failure rate might be.