The infant morality rate in the Soviet Union rose from 1971 to 1976 by one-third, and today is at least double and possibly triple the rate for the United States, according to a Census Bureau report written by two experts on the Soviet Union.

The report is based largely on official Soviet information and is ironic in view of statements by Soviet commentators a few years ago that health and infant mortality statistics are a "sensitive barometer" of a society's social and economic health.

The Soviet Union is the first developed nation to undergo such a "sustained reversal" of the infant mortality rate, according to the report.

The authors said the reason for the poor Soviet record include a high rate of alcoholism among pregnant women, a reduced percentage of the budget spent on health, a decline in breast-feeding without an adequate supply of quality formula and an inability to control influenza.

In 1971, the official Soviet rate of infant mortality -- deaths from any cause before the first birthday -- was 22.9 per 1,000 live births.

By 1976, the official rate had jumped to 31.1 deaths per 1,000, according to the study's authors, Dr. Murray Feshbach, chief of the Census Bureau's U.S.S.R. population and employment studies branch, and Dr. Christopher Davis of the Center for Russian and East European Studies, Birmingham, England.

This means that at least 38,704 more Soviet infants died in 1976 than if the 1971 infant death rate had still been in effect.

Over the same period, the U.S. infant death rate dropped from 19.1 to 15.1 per 1,000.

However, Feshbach, on leave from census and doing special studies at the Wilson Center of the Smithsonian here, said Soviet official statistics minimize the growth of the infant death rate because they don't count certain deaths that are included in the rate in the United States and other countries: deaths of some prematurely born infants, those born small and dying in the first seven days of life.

Feshbach said that if the Soviet infant death rate is adjusted to use the same definitions as in the United States, the 1976 Soviet rate is actually 35.6 per 1,000, or more than twice the United States' 15.1 per 1,000 rate.

Moreover, he said, although it isn't in the published study, he believes the Soviet rate since 1976 has continued to rise and is probably today around 39 to 40 deaths per 1,000 live births (using U.S. definitions). In the United States, by contrast, the rate has continued to fall, and was 12.9 per 1,000 for the 12-month period ending in March. Thus, the Soviet rate today is probably three times the U.S. rate.

Feshbach and Davis listed a large number of possible reasons for the rising infant birth rate in the Soviet Union.

Feshbach said the share of the Soviet budget going for health had dropped from about 6.6 percent in 1965 to 5.2 percent in the mid-1970s.

"The inability to cope with influenza is another major problem," her said. Influenza leads to pneumonia, which kills infants. He said the Soviets aparently didn't have sufficient vaccine to handle the many different types of flu. In some regions, he said, it is estimated that one-third to one-q uarter of infant deaths are the result of respiratory troubles due to flu.

Soviet women also "an incredible number" of abortions, about six per woman of child-bearing age, compared with one-half per woman in the United States. Some commentators believe this can cause health problems that may produce infant deaths later, when a woman carries the pregnancy to term.

Soviet commentators also mention alcoholism in pregnant women, leading to less healthy babies.