Teenage pregnancy cost the federal government nearly $22 billion in major welfare outlays last year, according to a study to be released today.

The Center for Population Options, a nonprofit organization that supports family planning, said the U.S. government in 1989 paid $21.55 billion in food stamps, Aid to Families with Dependent Children (welfare) and Medicaid benefits for families in which the first child was born when the mother was a teenager. About 9 million adults and children were in such families.

The study covered federal costs in 1989 for all families that started with a birth to a teenage mother, even if the birth occurred sever- al years ago. The study said

that about one-third of all families that started with a teenage birth has ended up on public assistance.

Judith Senderowitz, the organization's executive director, said the cost of government assistance may rise sharply in the future. "The sad fact is that for the first time in nearly 20 years, the teen birth rate is rising and with it the cost of government assistance," she said.

The birth rate in 1985 was 51.3 per 1,000 girls age 15 to 19, according to center figures. By 1988 the rate had climbed to 53.6.

The study said nearly a half-million babies were born to teenagers in 1988, some of whom already had one or more children. It said birth rates were increasing most sharply among those 15 to 17.

The study said the total national cost of teenage pregnancy in 1989 was higher than $21.55 billion, because certain federal outlays in addition to food stamps, AFDC and Medicaid -- such as housing subsidies, special education, foster care and day care -- were not included.

The study also did not include amounts states spent for families receiving AFDC and Medicaid benefits, which the authors said probably amounted to billions of dollars.

For families that receive public assistance after a teenage birth -- about one-third of all such families -- the federal government spends an average of $50,925 for food stamps, AFDC and Medicaid by the time the child reaches 20.