By the grisly measures of dying babies and declining life spans, American medical and demographic specialists are concluding that the Soviet Union is experiencing a persistent deterioration of health conditions that is unique among the industrialized nations.

"If we had their infant mortality rate," Surgeon General Julius B. Richmond said in a recent interview, "we'd be shocked." In virtually all the developed nations, deaths within one year of birth have consistently declined in the past decade, he noted, but in the Soviet Union the infant death rate has gone up. "Infant mortality is a pretty good proxy for social and economic conditions," said Richmond, a pediatrician and former medical school dean.

A similarly negative assessment of Soviet health conditions is offered by Murray Feshbach, a Bureau of the Census branch chief who specializes in analyzing Soviet population trends: "It seems as if the public health system in the Soviet Union is having difficulty coping with its problems." An analyst who did not wish to be identified put it more bluntly: "The health data simply reflect that some things may be cracking up there."

These dour assessments are necessarily based on incomplete and outdated information, because in 1971 the Soviets discontinued publication of life-expectancy statistics and, three years later, cut off figures on infant mortality. Until the blackouts, the Soviets had routinely followed the worldwide practice of regularly publishing this information, which is basic material for gauging national health trends.

Richmond, who has had extensive dealings with Soviet medical officials through intergovernmental health programs, says the Soviets brush aside inquiries about their health statistics and that he has no doubt that it's plain embarrassment that accounts for the concealment of the vital data.

Nonetheless, by supplementing the stale figures with evidence gathered from Soviet scholarly journals and other sources, analysts have concluded that the Soviet infant mortality rate has followed an unprecedented roller coaster pattern over the past 30 years, dropping from 81 per 1,000 in 1950 to 22.9 in 1971, and then rising to 27.9 in 1974 -- when publication ceased. Feshbach estimates the current rate at 35. The U.S. infant mortality rate, which has been steadily declining, is provisionally estimated for 1980 at 12.7 per 1,000.

While the difference between the two countries is astonishing, it may actually be greater than the numbers indicate -- 39 to 40, Feshbach believes -- since the Soviets alone exclude from their statistics several categories of premature live births that fail to survive the first week.

Meanwhile, life expectancy at birth, the other sensitive indicator of health conditions, is generally on the upswing among the developed nations, and now exceeds 70 years for American males. The figure for Soviet men is estimated to have declined over the past decade, from 66 to 63 years.

Quests for explanations of these statistics originally focused on the possibility that the surge in infant death rates simply reflected a decline in urban births, with the result that births in medically underserved rural areas accounted for a higher proportion of deliveries. But according to an analysis by Feshbach and Christopher Davis of the University of Birmingham, "it is evident that infant mortality is rising in cities in all of the major regions of the U.S.S.R."

As for the possibility that the gloomy figures result from no more than statistical quirks, the two analysts plausibly counter that Soviet demographic data are too extensive and long-running to allow for that simple out.

Richmond speculates that the infant mortality increase reflects a general deterioration in nutrition, along with official Soviet indifference to the health effects of smoking. Soviet consumption of vodka and home brew has sharply increased in recent years, and since this is mainly a male habit, it's a prime suspect in the decline in male life expectancy. But it may be catching on among women, too, which would be an element in the infant death rate.

Feshbach says he has recently come around to the view that, while environmental factors such as poor nutrition, alcohol and tobacco are major factors in the increased death rates, declining support for the Soviet health care system may be an additional cause. He points out that the health system's share of the state budget dropped 6.6 percent in 1965 to 5.2 percent in 1978. One effect, he says, has been a standstill in the quality of prenatal and infant care.

The Soviet Union's awesome numbers of missiles and tanks obviously exist at some price to that nation's domestic economy. What's little recognized is that the price includes a degree of health deterioration that is unknown in any other industrial country.