Washington's population dropped last year to 702,000 - a decline of 55,000 or 7.3 per cent sence 1970, according to new U.S. Census Bureau estimates.

Over the past six years, the bureau said, 77,000 more people have moved out of the District than moved in, for a net emigration of 10.2 per cent.

At the same time the city had 22,000 more births than deaths, but because the number of births was down sharply from what it had been during the 1960s, the rate of population loss was much more substantial.

The new census report also shows modest population gains in Maryland and Virginia.

Throughout the country, it said that overall population growth is down to its lowest levels since the 1930s, and that almost all of the increase since 1970 has been in the South and West.

In the most recent year covered by the report, from mid-1975 to mid-1976, the census bureau said the population of the United State rose by 1,6 million, but that only about 150,000 of the increase occurred in the Northeast and Midwest.

For the first time since colonial days, the census bureau said, slightly over half the nation's people now live in the South and West. In the 1970 census, these areas had 8 million fewer people than the North.

The new population figures are estimated for July 1, 1976, based on several complicated formulas using birth and death statistics, school enrollment data, and records from automobile registrations, income-tax returns, Social Security and immigration.

The drop in Washington's population us similar to that in many other big, older cities in the East and Mid-west.

Although the new figures do not give a breakdown by race, census experts and city officials believe that substantial numbers of blacks are now leaving the District for the suburbs.

Gan Ahuja, the District government's chief demographer, said the city's white population is still dropping, but at a much slower rate than in the 1950s and 1960s. The number of blacks, he said, is now falling at approximately the same rate as the number of whites so that the city's racial composition has stabilized at about three-quarters black.

Overall, the census bureau said, the city's population in mid-1976 was 10,000 fewer than it had been a year earlier.

One othe factor explaining the city's population loss. Ahuja said, is the virtual end of migration to Washington from the rural areas and small towns of the South.

"In the 1960s, there was a very high in-migration from the South," Ahuja said, "but in the 1970s it's not occurring. This year we will have some from Georgia, of course, but that's a small number."

The movement of young whites into Capitol Hill, Mount Pleasant, and east of Dupont Circle also has led to a population loss, Ahuja said. He explained that many of the old houses they buy and restore had much larger number of low-income blacks living in them as renters.

Ahuja said the decline in births, part of a national trend, has hhad a powerful impact here. In 1975, the most recent year for which data is available, Washington had only 9,800 births, compared to about 15,000 in 1970. The number of deaths fell much less, so that the surplus of births over death, which demographers call natural increase, dropped from 6,300 a year to 2,000.

The new census report estimates that Maryland's population grew by 22,000 last year to 4,144,000, while Virginia'population rose by 51,000 to 5,032,000.