Baby boomers finally are having babies, immigrants are crowding into small apartments, secretaries can't afford to pay the rent without roommates -- and the result, demographers say, is that the suburban Washington household is growing again.

The average American household has been shrinking for decades, and the trend sharpened in the 1970s when children of the post-World War II baby boom grew up. They moved into their own homes and stayed single longer than earlier generations. The nation's divorce rate rose, sending more people out on their own.

Recently released preliminary numbers from the 1990 census show that the decline is leveling.

In Alexandria and the counties of Fairfax, Montgomery and Prince George's, households were smaller in 1990 than they had been in 1980, but the pace of decline slowed in the middle of the decade and household size began gaining again, demographers said.

For example, the average household size in Fairfax was 2.83 in 1980 and had fallen to 2.73 in 1985. By 1990, the number had risen to 2.75. Even a small shift is significant, demographers say.

In Arlington, the average household was larger at the end of the decade than at the beginning: 2.12 people per household, compared with the rock-bottom 2.07 of 1980.

"That's really surprising," said Margaret Simkovsky, research coordinator in Arlington's planning division. "We thought we were down to 1.99."

Household size reflects patterns of American life more than any other statistic, and the reversal shows that those patterns are changing locally and across the nation, demographers say. They caution, however, that families will not grow back to "Eight Is Enough" size.

"I don't think there's any sign we're going to go back to the three-child family any time soon," said George Grier, of the Greater Washington Research Center.

Grier's research indicates that between 1980 and 1986, the percentage of area households with one person living alone declined while the percentage of two-, three- and four-person households increased.

Although more detailed census figures that could more fully explain the reasons for the change in household size will not be available until next year, demographers believe a major explanation is that marriage rates are rising, the divorce rate is down and baby boomers -- childless for years -- are starting families.

The number of births in Montgomery County has nearly doubled since the mid-1970s, making for a kindergarten class that is larger than the number of graduating seniors.

Fairfax County's birth rate in 1988 -- the number of babies born per 1,000 people -- was higher than in any year since 1970. Fairfax Hospital, which delivers more babies than any other hospital in suburban Washington, brought 8,912 babies into the world last year, up one-third from the beginning of the decade.

The increase in births has come because of delayed child-bearing and because the suburbs continue to attract young families and are growing rapidly. Montgomery County's population expanded by 30 percent over the past decade, and Fairfax County's grew by 37 percent.

Demographers say this reflects the huge numbers of the baby boom generation, who have dominated the nation's social trends at every stage of their lives. Washington is a magnet for baby boomers, second only to Denver in the proportion of its population made up of people born between 1945 and 1960, according to a recent report by the Population Reference Bureau.

The increase is intensified by a jump in births among women approaching the end of their fertile years. Nationally, the most dramatic expansion in births in the past decade was among women ages 35 to 39. Montgomery County, a magnet for well-off baby boomers, has the highest birth rate in Maryland for women in their thirties.

Eileen Andreoli, head of the childbirth education program at Fairfax Hospital, said a majority of the couples in some of her classes are in their mid-thirties to forties.

"They made career choices and delayed child-bearing," she said.

More children will translate into the need for more schools. Grier reported recently that there are more than a million children in the Washington area -- the largest number ever -- and that will require the equivalent of as many as 180 new elementary schools in the region over the next decade.

Much of the new school construction will be outside the Capital Beltway, but many closer suburbs are expanding existing schools.

Another reason for the change in household size, many local officials say, is a burst of immigration that is bringing in large families with members from several generations to area suburbs. Simkovsky said there are reports of a "fair bit of overcrowding" in some apartment complexes, especially among Hispanics.

"Often it's part of a family that, because of the high cost {of rent}, has to live with another family," said Evelyn Dominguez, supervisor of the Arlington Office of the Hispanic Committee of Virginia, a private group.

Others, too, are sharing quarters in a region that ranks near the top of the nation in its cost of housing. The Census Bureau reported that more adult children are moving back to the parental nest to save money, roommate-matching services are growing and a new breed of town house -- with two master bedrooms for people who need to share expenses -- has sprung up in the past decade.

"People who are making what we always thought were decent salaries can't afford to live alone," said Marilyn Wissoff, a housing counselor with Operation Match, a roommate referral service run by the Montgomery County government that signs up 100 clients a month.

A secretary earning less than $20,000 a year cannot afford the rent on a one-bedroom apartment in the county, which averages $615, she said.

Only in the Washington suburbs outside the Beltway -- in Loudoun, Prince William, Anne Arundel and Howard counties -- did the average household size decline at a greater rate than the national average, although those households remain larger than the U.S. average, according to the census numbers.

Grier said the decline in the outer suburbs could mirror those counties' transformation from more rural locales where large families are common, and he predicted that household size would rise in the 1990s as those areas continue to attract young families.

The preliminary census figures also showed that the District's average household size -- which was smaller than the national average -- shrank even more than the national average during the decade, but city officials say they do not accept the preliminary numbers and are filing a formal challenge.