For the first time in more than three decades, the number and proportion of out-of-wedlock births decreased last year in the District of Columbia.

The decline occurred among both black and white city residents and among child-bearing women of virtually all age groups, according to a new D.C. government report.

Overall, births to unmarried women accounted for 56.7 percent of the 9,378 children born to city residents in 1982, the report said, compared with 58.8 percent of the 9,332 children born a year earlier.

The last time the proportion of out-of-wedlock births fell in the city was in 1949, when they accounted for 11 percent of all births.

Nationally, the proportion of births to unmarried women reached 18.4 percent in 1980, the most recent year for which statistics are available. This continued a rise that began around 1950 but accelerated sharply during the 1970s.

Warren Morse, acting chief of the city's health research and statistics branch, said the prime factors in Washington's 1982 drop in out-of-wedlock births were the continued decrease in births to D.C. teen-agers, among whom more than 90 percent of births are out-of-wedlock, and a continued upsurge in births among married women older than 30.

"There's a change in the age structure," Morse said, "with fewer teen-agers around living in the city . At the other end (older than 30) there are a lot of women who postponed having children and are catching up for lost time. We may be seeing a spurt in a catch-up group having children now. But we can't be sure from one year's data how long it's going to last."

Although nationwide data is not available, Paul Placek, a demographer at the National Center for Health Statistics, said the decline in D.C. may be a harbinger of national trends, stemming from the "baby bust" of the 1960s and 1970s.

"With fewer teen-agers, you can predict that there may be a more stable society," he said. "Fewer out-of-wedlock births, a reduction in the crime rate, better performance in schools. I think a lot of this is happening."

But data for 1982 show continued increases in out-of-wedlock births in Maryland and Virginia, although the proportion in both states remains much less than in the District. In Maryland, according to provisional figures from the state Health Department, the proportion of out-of-wedlock births rose to 27.7 percent in 1982, up from 27.2 percent a year earlier and 25.7 percent in 1980.

In Virginia, 19.6 percent of births in 1982 were to unmarried women, compared with 19.4 percent in 1981 and 19.1 percent in 1980.

Out-of-wedlock births have continued to increase in all of the Washington suburbs except Alexandria, which shares many of the District's social characteristics. Over the past two years the proportion of out-of-wedlock births declined in Alexandria from 28.6 percent to 27.1 percent.

Among blacks in D.C.--who constitute about 70 percent of the city's population and accounted for 82 percent of its births last year--the number of children born out-of-wedlock fell by 109 in 1982 to 5,066, while births to married women rose by 70 to 2,644.

Overall, 65.7 percent of children born to black D.C. residents last year were out-of-wedlock, compared with 66.8 percent out-of-wedlock in 1981. The most recent national figure shows 55.3 percent of black births were out-of-wedlock in 1980.

Among D.C. whites, births to unmarried women fell by 58 last year to 250, accounting for 15 percent of all white births in the city compared with 19.5 percent a year earlier. Nationally, 11 percent of white births were out-of-wedlock in 1980.

According to D.C. government figures, the overall proportion of out-of-wedlock births among D.C. residents rose from 11.4 percent in 1950 to 19.8 percent in 1960, and 40.8 percent in 1970. It exceeded 50 percent for the first time in 1975, a proportion that out-of-wedlock births among D.C. blacks first reached in 1972.