Fear of getting AIDS is prompting nearly one unmarried American woman in three to change her sexual behavior, a government report estimates.

Studies have found that most people know how the AIDS virus is spread, but a survey by the National Center for Health Statistics was the first government effort to determine adjustments women made from that knowledge."People's habits are not easy to change and, as with changes in cigarette smoking, change in sexual behavior follows knowledge with frustrating slowness, but at least it does occur," said Dr. James Mason, assistant secretary for health in the Department of Health and Human Services.

The report, based on a 1988 survey and released Tuesday, said 31 percent of unmarried women who had ever had sexual intercourse had made at least one behavior change to reduce their risk of becoming infected with the AIDS virus.

Dr. William Roper, director of the federal Centers for Disease Control in Atlanta, said he was encouraged by the report but that more AIDS education is needed.

AIDS is most often spread among adults by needle sharing when one of the drug users is infected with the virus and by sexual intercourse, both homosexual and heterosexual.

The survey pointed up a weak spot in the education and prevention effort - that nearly one-third of the women surveyed did not know that AIDS can be spread by people who are infected with the virus but do not have the disease itself. This represents more than 8 million unmarried women, the report said.