Mary Bradley Meyer, 65, an associate professor of epidemiology at the Johns Hopkins University School of Hygiene and Public Health and the author of noted studies on the effects of smoking during pregnancy and other topics, died of cancer Thursday at her home in Baltimore.

Mrs. Meyer ws born and grew up in Boston. She graduated from Vasar College in 1937 and the moved to Baltimore, where she taught at the Garrison Forest School. She undertook graduate studies in bacteriology at Johns Hopkins in the early 1940s. These were interupted by World War II and the raising of her family. In the late 1950s, she resumed her academic career and earned a master's degree at the John Hopkins School of Hygiene and Public Health in 1958.

In the same year, she was appointed an assistant in the department of epidemiology there. Four years later, she published her first major study. The subject was mumps and it provided new information on the incidence of the disease in different age groups, its incubation period and the way its spreads in families and schools.

Mrs. Meyer also carried out a number of studies on pregnancy, including the ways in which prenatal exposure to radiation affects the development and fertility of human fetuses.

Her best-known work was on smoking during pregnancy. It has been included in several editions of the U.S. Surgeon General's Report on the Health Consequences of Smoking.

At a meeting of the American Association for the Advancement of Science in San Franciso in January 1980, Mrs. Meyer reported that heavy maternal smoking can cause spontaneous abortions and stillbirths and may account for as many as 35 percent of all premature births in the United States. Moreover, she said, infants born to mothers who are heavy smokers may be smaller at birth, grow at a slower rate and be more prone to respiratory and other illnesses than children born to non-smokers.

Mrs. Meyer said that while the connection between smoking and prenatal and postnatal problems is clear, the why of the matter is somewhat in doubt. But she observed that the nicotine in tobacco causes erratic changes in the mother's blood flow. And she pointed out that the carbon monoxide from cigarettes binds itself to the hemogloblin in the bloodstreams of the mother and the fetus, thereby reducing the amount of oxygen the unborn child receives.

Mrs. Meyer, who continued her work until her death, was appointed a research associate in the Johns Hopkins School of Hygiene and Public Health in 1962, an assistant professor in 1967 and an associate professor in 1973. She was a member of the school's faculty executive committee and the committee on scholarships and was a former chairman of the committee on admissions.

She published numerous papers in professional journals. She was a member of the American Association for the Advancement of Science, the Society for Epidemiologic Research and the Radiation Research Society.

In private life, Mrs. Meyer was a founder and member of the board of the Baltimore Choral Arts Society. Until recently, she sang in its chorus.

Her marriage to Dr. Eugene Meyer III ended in divorce.

Survivors include one son, Eugene Bradley Meyer of Lincoln, Mass.; three daughters, Ruth Meyer Guffee of West Haven, Conn., Anne Meyer of Wenham, Mass., and Elizabeth Ernst Meyer of Edgartown and Cambridge, Mass.; one sister, Mrs. Charles A. Janeway of Weston, Mass., and five grandchildren.