WASHINGTON (AP) _ America's population may decline for the first time after two centuries of steady growth, due to declining number of women of childbearing age, the Census Bureau says.

After increasing from nearly 4 million in the first census in 1790 to nearly 247 million people today, the population could peak at 302 million in 50 years before starting to dwindle, the bureau says.

That's because the massive post-World War II Baby Boom generation is now headed out of the prime childbearing ages, meaning fewer new babies can be expected in the future, Gregory Spencer, a bureau demographer, said Tuesday.

A much smaller group of young women is coming along into the childbearing ages, Gregory said.

Already the number of women aged 18 to 34 has begun to decline, the study noted. Peaking at 35.3 million in 1985 it slipped to 35.2 million by 1987 and is expected to drop to 34.7 million by 1990 and by 2080 to be below 30 million.

''After 1995 the population may grow more slowly than ever before - more slowly than even during the Great Depression of the 1930s,'' Spencer reported.

The peak in 2038 and subsequent decline is the middle projection of a series of some 30 different possibilities included in the study.

The lowest growth estimate would have the population peak at 264.5 million in the year 2020 before starting to decline, while the high-growth scenario would have the nation at 501 million and still growing in 2080.

While wary of calling any series ''most likely,'' Spencer said that in general, when people ask what the population will be like in the future, he gives them the middle series of projections.

The Bureau compiles the 30 different sets of estimates in an attempt to account for varying possibilities in birth rates, death rates, differences between races and ethnic groups, immigration and so forth.

During that coming half-century the middle series of projections calls for an overall increase of about 56 million people, nearly the same amount by which the U.S. grew during the last 25 years.

After that, shrinkage should occur, although it will be ''really quite modest,'' with the population falling by only about 10 million over the subsequent 42 years, Spencer said.

''So, obviously, it wouldn't take very much to make this not happen,'' he said.

The report also discloses that the nation's elderly population, which has attracted so much attention in recent years, is expected to grow more slowly after the mid-1990s, and then accelerate again after 2010.

That slowing is expected because the small generation born in the Depression and war years of the 1930s and 1940s will be moving into this age group.

But once the post-war Baby Boom reaches that plateau the elderly segment will resume the rapid growth it has seen in recent years.

Other findings of the new report include:

-The share of the population under age 35 may never again be as large as it is now - 55 percent. That percentage is expected to drop to 48 in 2000, 46 in 2010 and 41 in 2030.

- The white population may begin to decline in 40 years while the population of black and other races is expected to continue increasing.

- By the year 2030 the population could include as many as 32 million immigrants who entered the country after 1986, and their descendents - 12 percent of the population.

Here is a rundown of the projected U.S. population in coming years, based on the middle set of estimates. The Census Bureau estimated the U.S. population at 246,879,546 as of Jan. 1 of this year. Year Projection 1990 250,410,000 1995 260,138,000 2000 268,266,000 2005 275,604,000 2010 282,575,000 2015 288,997,000 2020 294,364,000 2025 298,252,000 2030 300,629,000 2035 301,725,000 2038 301,881,000 2040 301,807,000 2045 301,100,000 2050 299,849,000 2055 298,369,000 2060 296,963,000 2065 295,744,000 2070 294,642,000 2075 293,500,000 2080 292,235,000