THE INFANT MORTALITY RATE in this country is now dropping rapidly, a tremendously heartening trend. It reflects improved health care, in the conventional sense of the term, but it also reflects a good many other things going on in American life. The recent statistics on infant deaths provide commentary on social history.

The death rates among babies have always been higher in the United States than in most other rich countries. It's a matter of particular reproach when you consider the advanced state of medicine here. But it takes more than good doctors to get the rate down. The decline in infant deaths has followed an irregular pattern over the years that offers some hints of what may be happening.

During World War 1, 100 of every 1,000 babies died before they were a year old. The number fell continously through good times and bad until, in 1950, it was down to 29. Suddenly the steady progress ended. For the next 15 years, improvement was halting and of medical knowledge, since rates in other countries continued to fall. But, equally suddenly, in the late 1960s the American rate started briskly downward again. Last year it was 15 deaths for every 1,000 babies born, and this year will probably be less than 14. Nobody has any precise explanation of this stop-go sequence, but some of the reasons seem fairly clear.

For one thing, high birth rates and big families tend to mean higher proportions of infant deaths. The 1950s were, of course, the years of baby boom. The 1940s and 1950s were also the years of the great surge of blacks from the rural South into the ghettoes of the great northern cities. For a lot of those families it was a migration into a far less healthy life. In the 1950s, the gap between death rates for whites and black babies widened significantly. Then, in the latter 1960s, things began to get better.

The rapid drop in birth had something to do with it. Wider access to abortion meant that fewer babies were unwanted. Doctors were beginning to be able to identify, long before birth, congenital defects that would prevent babies from surviving. Hospitals were learning to centralize maternity services and special care for infants who needed it.

But there was also the food stamp program, which went into operation in 1964. While no statistician's pencil is sharp enough to identify precisely the separate effects of food stamps from all of the other influences on infant health, the country is now putting some $5 billion a year into the nutrition of people who previously were, by any standard, inadequately fed. Some of those people were pregnant women. Medicaid went into business in 1966. Again, it's hard to show a direct cause and effect. But it's clear that more women now see doctors during pregnancy.

Perhaps an even more powerful influence - one that underlies much that has changed since the 1950s - is the dramatic rise in the general educational level of the American population. In 1950, the average American women had a ninth-grade education. By the late 1960s, the average woman had finished high school.

The record suggests that while the quality of medical care is important, it is hardly the only important factor in raising the survival rates for newborn babies. Public policy in housing, schooling, nutrition and perhaps even environmental protection has an impact. The data from other advanced countries keep offering the United States a useful reminder that it has a long way to go before it truly presses the limits of medical capability to help every child who has a chance to live.