Americans were married, had babies and died at increased rates last year while the infant death rate plunged and the divorce rate leveled, according to a report released yesterday.

Nearly 11 of every 1,000 Americans tied the knot in 1980 in a record 2.413 million weddings, the National Center for Health Statistics found. The numbers are the highest in history, and the percentage is just a hair below the peak year, 1972.

What's fueling the heartwarming increase in nuptial activity? "It's really divorces that are contributing to the marriage rate," explained the center's Richard Klain.

"The rate of remarriages is double that of first marriages, which has stayed fairly constant. And divorced people have the highest marriage rate of any group--higher than widowed people, higher than singles. So it's the same people," he said.

June weddings are still favored, the center found, with 278,000 ceremonies in that month compared with 140,000 in January, the least popular month.

Last year's 1.182 million divorces represented a 1 percent increase above the 1979 record. But the divorce rate stayed steady at 5.3 per 1,000 people.

As the marriage figures increase, Klain said, divorces also should increase. In turn, marriages increase.

The number of births climbed for the fifth consecutive year, up 4 percent from the previous year, for a 1980 total of 3.598 million. The birth rate of 16.2 per 1,000 people reached its highest level since 1971, when the rate was 17.2.

The fertility rate was the highest since 1973, at 69.2 births per 1,000 women aged 15 to 44.

The largest increase in births is among women aged 30 to 34 who had postponed starting families at a more traditional age, Klain said. Also, the postwar baby boom generation is swelling the number of women of childbearing age at the same time as the actual increase in the rate at which women are having babies.

The infant death rate was the lowest ever recorded in the United States, the report showed. About 45,000 infants died before their first birthday in 1980, or 12.5 for every 1,000 live births. That was down almost 4 percent from 1979.

There were 250 deaths of women due to complications of pregnancy, childbirth and in the six weeks after delivery--down from 270 in 1979. The maternal death rate of 6.9 deaths per 100,000 live births was down dramatically from the 7.8 rate of 1979, compared with 21.5 in 1970 and 83.3 in 1950.

At the same time, life expectancy at birth fell slightly for the first time in 12 years, from 73.8 to 73.6 years. The report blamed the drop on flu epidemics. Flu and pneumonia were the sixth-highest causes of death in 1980, a 14 percent increase above 1979.

Death rates went up for all racial groups and for both sexes, but rose higher for non-whites than for whites and higher for women than for men.

White females born in 1980 could expect to live 78.1 years, and other females could expect to live 74 years. White males could expect to live 70.5 years; for other males, expectancy was 65.3 years.

A total of 1,986,000 persons died last year--a death rate of 892.6 per 100,000, up 3 percent from 1979. Adjusted to account for the growing proportion of elderly and other population changes, the increase was just 1 percent.

Heart diseases and cancer remain the leading causes of death, the report said.

With births outnumbering deaths, the nation's population showed a natural increase of 7.3 persons per 1,000.