Prince George's County, once Washington's fastest growing suburb, is continuing to lose population as the movement of black families into its neighborhoods is failing to offset sharp losses in white population.

New Census Bureau estimates show that the county's population fell by 7, -200 in the latest 12-month reporting period and by 31,500 since 1972 when its growth peaked. The bureau now places the county's population at about 662,200 -- about the same as it was at the start of the 1970s.

Studies by the Maryland State Health Department show major changes in the county's racial composition during the decade, with the number of blacks in the county more than doubling between 1970 and 1977 and the number of whites falling by 20 percent. In 1970, blacks accounted for 15 percented of Prince George's resident; by 1977, the health department estimated that one third of the residents were black.

The Census Bureau's estimates for mid-1978 also showed continued, substantial growth in Fairfax County, where much of the region's new housing is being built, and more modest growth in Montgomery County.

The most rapid development area remained the fringes of the metropolitan area, as it has been throughout the 1970s. For example, Spotsylvania County around Fredericksburg, Va., has grown by 80 percent -- 29,600 -- since 1970. Howard County, Md., which contains the new town of Columbia, has grown by 87 percent -- 116,600 -- since the start of the decade.

Both Loudoun and Prince William counties, just inside the official metropolitan area, continued to grow significantly last year, but at a somewhat less rapid pace than they had during the first half of the 1970s.

The close-in suburbs of Arlington showed continued population losses, but the decline from 1974 to 1978 was just 5,000 compared to a 20,000 loss during the previous years. The mid-1978 population in Alexandria, another close-in suburb, was 400 more than it had been a year earlier, reversing declines that were recorded each year since 1970.

The new figures are esitmates for July 1, 1978. They are based on several complicated formulas using birth and death statistics, school enrollment data, information from income tax returns and Medicare records.

A similar estimate for the District of Columbia was issued in January, along with figures for the 50 states. That report put the city's population at 674,000, about 11 percent less than in 1970.

Overall, the new estimates indicate there was been virtually no change in the total population of the Washington area since 1975 in contrast to the population boom of the 1960s and slight increases during the first half of the 1970s.

Underlying the new pattern have been two central factors: a sharp decline in births, which occurred nationwide, and a shift in a magration trends. During the 1970s, about 95,000 more people have left the Washington area than moved into it.

The turnabout was sharpest in Prince George's County, which was the region's fastest growing jurisdiction in the county's racial composition.

Since the early 1970s, "the black families coming into the county have been smaller than the white families moving out," said George W. Grier, a demographer who heads a local consulting firm. "Most of them are upwardly mobile people who are younger than the whites [moving out], and are subject to the lower birth rates people have these days.

"The change certainly doesn't mean a decline in the county," Grier added. "Quite the opposite. It could mean fewer burdens in terms of kids in the school system, and a larger wage-earning population because a lot of [the blacks] are in two-earner households."

Grier said a significant factor in the loss of whites in Prince George's was the court-ordered busing program for school desegregation, which started in January 1973.

Frederick J. Cavanaugh, chief of the local area estimates branch of the Census Bureau, suggested that the slight growth in Montgomery County -- an increase of 3,300 last year and 53,000 since 1970 -- reflects the county's continued ability to attract older, more affluent families. Such families have dominated Montgomery's population since the 1950s, "and they're still coming," Cavanaugh said.

Population growth in Fairfax has been much greater -- 12,000 last year and 90,000 since 1970 -- because Fairfax has a wider range of new housing prices than Montgomery, Cavanaugh said.

"They're attracting a lot of young families and older families too," he remarked. "There are still parts of Fairfax that are very rural -- lots of space to build new homes."

In contrast, Cavanaugh and Grier said, the built-up areas of Arlington and the District of Columbia have lost population as families have been replaced by single adults childless couples, attracted by proximity to jobs.