For the first time in 20 years the number of new households in the United States showed no significant increase over the year before, the Census Bureau will report today.

The census report called the new findings "a dramatic departure from the household trends of recent years," when the net increase in the number of new households exceeded 1.6 million annually.

"One of the reasons this is so surprising is that we would expect as the 'baby boom' generation grows up that there would be continued growth in the number of new households," said Steve Rawlings, a demographer with the bureau's Family Statistics Branch.

He suggested that the drop might be the result of the recent recession and high interest rates, which have discouraged home buyers, and the declining divorce rate. He cautioned against drawing firm conclusions based on a single year's change, however.

"Baby boom" is the term demographers give the unusually high U.S. postwar birth rate between 1949 and 1964. The survey counted 83.9 million households as of March, 1983, an increase of only 400,000 since March, 1982. More than 20.5 million new households were formed between 1970 and 1983, the study said.

"Since nothing's changed in the demographic engine that should be fueling continued growth in the number of new households," Rawlings said, "this suggests economic difficulty--people don't have the means to set up a household on their own, living at home with their parents longer, for example.

"Another likely factor," he said, "is that in the 1970s we had a tremendous increase in the divorce rate, which contributed to the proliferation of new households as people went their separate ways.

"But recent figures show the divorce rate has stabilized and could finally be declining," he said.

And the number of cohabiting couples remained stable this year, he said, after rising nearly 300 percent between 1970 and 1980.

About 73 percent of American households--61.4 million--are maintained by families, defined as two or more related persons. These include 49.9 million households where both husband and wife are present, 2 million with no wife present and 9.5 million where the husband is absent.

Since 1970, the number of family households headed by women alone grew from 5.5 million to 9.5 million.

The study is based on the March, 1983, Current Population Survey, a monthly census of 65,000 households throughout the nation. The surveys are used to determine unemployment levels and other population trends, including voting behavior, income and education.