Breaking a trend of the past decade, the infant mortality rate in the Boston area increased by 32 percent in 1985, a trend concentrated solely among Black infants, state officials said yesterday.

Statistics released yesterday by the Massachusetts Department of Public Health show that whereas the mortality rate of white and Hispanic infants decreased between 1984 and 1985, the mortality rates of Black infants rose in 13 cities and towns across the state. This increase of deaths among infants under the age of one contrasts with the general decrease of national and state infant mortality of the past decade, said Department of Public Health spokesman John D. Stobierski.

Cambridge had a more than 50 percent increase in its infant mortality rate, which was reflected mostly in the Black community, Stobierski.

Stobierski said that Massachusetts has "generally been below the national average for infant mortality." The national date for the infant mortality rate of 1985 is not yet available.

Officials attribute the decrease of mortality among white infants to better prenatal care, such as improved intensive care units.

But experts are not certain what has precipitated the rise of infant mortality in Massachusetts. Most said that it is probably a result of the decrease in social services.

"What we're seeing is a worsening of the health status of poor people" said Dr. Michael Weitzman, medical director of parent and child health services at Boston's Department of Health and Hospitals. "Blacks are more likely to be poor than whites. The higher mortality rate reflects the failure of safety programs to compensate for people who live in poverty."

"I don't know why this is happening," said Dr. Milton Kotelchuck of Harvard Medical School, "but the Black community is more heavily dependent on public-supported prenatal care, and the cut-backs on public institutions have hurt them."

Director of the Massachusetts Health Action Alliance, Susan T. Sherry, said that the Alliance has been working to counteract what it sees as the main causes of the infant mortality increase. She cited "inadequate physician participation in Medicaid" and the increase of uninsured people as possible causes for the increase of infant mortality. "People don't get preventative care if they're uninsured," she said.

Officials said a task force, which suggested ways to improve prenatal care in 1985 after the infant mortality increase of 1982, will reconvene in order to suggest further ways to improve access to prenatal care.