The number of legal abortions obtained by Washington residents last year totaled 12,945, about one third more than the city's 9,635 births, the District of Columbia government reported yesterday.

The Department of Human Resources, which released the statistics, said that about 7,400 of the abortions - or 57 per cent - were paid for by the federal Medicaid program, which stopped making such payments a month ago.

Although the city has agreed to continue paying for abortions for low-income women until Sept. 30, DHR director Albert P. Russo said yesterday that no decision has been made yet on extending the payments after that.

Russo said he personally favors the abortion payments as a matter of "public good." However, Mayor Walter E. Washington and the City Council must decide the matter.

Officials of Planned Parenthood and federal officials who compile abortion statistics said Washington is the first major American city where abortions performed on residents outnumber births.

In 1975, when the District first reported more abortions on residents than births, the city government said the excess of abortions over births was fewer than 100. For 1976, it said, there were 3,310 more abortions here than births.

However, Warren Morse, the DHR statistician in charge of compiling the reports, said the abortion figure for 1975 was seriously understated because five clinics and hospitals submitted incomplete data. Morse said the 1976 data are complete.

He said he now believes the number of abortions obtained by D.C. women in 1975 was about the same as last year's 12,945, but he added that there probably will be a considerable drop if low-income women can no longer get free abortions.

In a separate report yesterday, DHR said 1976 was the second year in a row in which more than half the births to Washington residents were out of wedlock.

Overall, DHR said, 5,065 children were born to unmarried women here in 1976, compared to 4,570 births to women who were married. The proportion of out-of-wedlock births was 52.6 per cent, compared to 51.2 per cent in 1975.

According to officials of the National Center for Health Statistics, Washington is the first large American city where the majority of births are born out of wedlock. However, in several big cities the proportion of out-of-wedlock births now exceeds 45 per cent, including Newark, N.J., where it reached 49.2 per cent last year.

According to the new D.C. government figures, the total number of births recorded in Washington declined last year, as it has for more than a decade, but the year-to-year drop - just 121 - was the smallest since 1970.

The number of births to white D.C. residents actually rose by 58 - the first slight increase for whites in three years, but the white total was only 1,342 - just 13.9 per cent of the citywide toatal.

Morse said he believes the number of births to while D.C. residents now is "bottoming out," while the number of births to blacks is continuingto decline slightly, although he said there may be a small upturn during 1977 for both races, which would be part of a nationwide pattern.

Morse and other city officials declined to speculate on the reasons for the exceptionally high number of abortions here.

But Jeannie Rosoff, director of the Washington office of Planned Parenthood said one factor probably is the widespread availability of abortions in the District.

According to data compiled by Planned Parenthood, the abortion rate among blacks nationally is about double that for whites and Rosoff said that could be another factor in explaining the high abortion rate here.

In 1975, the last year for which nationwide figures are available, the number of abortions among whites amounted to about 48 per cent of black births.

The national data includes states where abortions still are difficult to obtain despite the Supreme Court decision in 1973 legalizing abortions throughout the country. However, in New York City, where abortions are as easily available as in Washington, the number of abortions among blacks last year exceeded the number of births by 7 per cent.

Dr. Jean Pakter, director of maternity services and family planning for the New York City health department, said the high ratio of abortions among blacks is probably accounted for by their high incidence of poverty and the hig proportion of teen-agers in the black population. She said both factors are tied to less effective use of contraceptives, which in turn leads to more pregnancies and the more frequent decision to have an abortion.

Nationwide, about 14 per cent of all births in the United States were out of wedlock in 1975, a proportion that has been increasing steadily since 1960. Among blacks, 48.8 per cent of all children were born out of wedlock in 1975. Among whites, 7.3 per cent were out of wedlock.

In Washington, the proportion of out-of-wedlock births among blacks last year was 58.9 per cent; among whites it was 13.3 per cent.