Teen-age pregnancy cost the state and federal governments at least $16.65 billion in welfare outlays in 1985, according to a study released yesterday by the nonprofit Center for Population Options.

Center Executive Director Judith Senderowitz, calling the rates of teen-age pregnancy "staggering," said, "These numbers bring home that teen-age pregnancy is everyone's problem, not merely the pregnant girl and her family's problem."

In fact, the findings of the study are consistent with earlier studies by others on the same issue. The same costs were estimated at $8.55 billon in 1975, which would translate to more than $16 billion in 1985 dollars.

Generally, students of welfare say, teen-age mothers, particularly unmarried ones, are more likely to end up on welfare, to need Medicaid and to drop out of school than women who start having children at a later age.

Since the end of the baby boom, the rate of births to teen-agers has actually been dropping. But the percentage of teen-age mothers who are unmarried has more than tripled over the past 25 years, from 15 percent in 1960 to 54 percent in 1983.

The new study, performed by sociologist Martha Burt, estimated the 1985 cost of three major welfare programs for families with children up to 20 years old in which the mother had her first baby while in her teens, whether married or unmarried.

Burt calculated that the combined federal-state costs of Aid to Families with Dependent Children (AFDC) for such families totaled $8.32 billion in 1985, that the combined Medicaid costs for such families amounted to $4.91 billion and that the food-stamp program costs were $3.42 billion.

Burt did not include the cost of other programs that also give benefits to such families -- such as Head Start, the Women-Infant-Children special food program, subsidized housing, child welfare, special education and foster care -- but said the total costs of teen pregnancy would clearly have been even greater than $16.65 billion if she had been able to assemble the figures needed to include these other programs.

More than 1 million teen-age girls become pregnant each year but, because of abortions and miscarriages, only about 500,000 give birth. More than half the number who give birth are unmarried, and about three-quarters are having their first baby.

In addition to calculating the 1985 single-year cost, Burt looked at the costs of teen-age pregnancy from another perspective, estimating that each baby born in 1985 as the first child of a teen-ager would cost the public $15,620 per child in 1985 dollars over the next 20 years. These figures included AFDC, Medicaid, and food stamps, as well as housing and social services programs.

And she calculated that the combined benefit costs of all first babies born to teen-age women in 1985 would come to $6.04 billion over the next 20 years.

Also yesterday, the House Ways and Means public assistance subcommittee began a series of hearings on teen-age pregnancy and welfare problems with a showing of the CBS TV broadcast "The Vanishing Family -- Crisis in Black America." A panel of experts, including Barbara Blum, president of the Manpower Demonstration Research Corp.; Douglas Glasgow, vice president of the National Urban League; Marian Wright Edelman, president of the Children's Defense Fund and Joyce Ladner, Howard University professor and chairman of D.C. Mayor Marion Barry's special panel on teen-age pregnancy, said that many of the most serious problems are related to the lack of jobs and job training for young black males who father children out of wedlock and have no jobs to support them.

They said that in addition to programs to encourage young mothers to complete school and find jobs and to avoid pregnancy, there must be a major effort to train and employ young black males.