The proportion of two-parent family households has dropped sharply since 1970 and now makes up just 26 percent of all U.S. households, a Census Bureau study reported Tuesday.

At the same time, however, the bureau reported that while the number of single parents increased by 2.8 million in the past decade to 9.7 million, the numbers grew more slowly in the 1980s than in the previous decade.Still, single parents accounted for 28 percent of the 34.7 million one- and two-parent family groups with children under 18, the report said.

According to the bureau's report, based on its March 1990 Current Population Survey and not the April decennial census, only 26 percent of households consisted of a married couple with children under age 18, compared with 31 percent in 1980 and 40 percent in 1970.

There were about 93.3 million households in March 1990, up 12.6 million households from 1989. Households, for the Census Bureau, are all occupied housing units, and although most households involve some type of family living arrangement, many do not.

Of the 93.3 million households in 1990, family households - including single parents and parents with no children or children over 18 - accounted for 71 percent of the total, down from 74 percent in 1980 and 81 percent in 1970.

About 52.3 million households consisted of married-couple families while another 10.9 million families were headed by a woman with no husband present. And 2.9 million families were headed by a male housholder with no wife present.

The report also showed there were 27.3 non-family households in 1990 but said the growth in the number of such households has "slowed significantly, dropping from an annual rate of 5.,7 percent in the 1970s to 2.5 percent in the past decade."

It also found the number of persons per household averaged 2.63 people in 1990, down from 2.76 in 1980 and 3.14 in 1970.