American life expectancy has fallen for the first time since 1968, according to an insurance company study. It saisd an American born in 1980 can anticipate living 73.8 years, or about 3.5 months less than one born in 1979. But life expectancy remains three full years greater than in 1970. For a newborn boy, life expectancy was 70.1 years, compared to 70.2 in 1979 and 67.1 in 1970. A girl could look forward to 77.5 years of life, compared with 77.9 for one born the year before and 74.7 for her 10-year-old sister. The figures, compiled by Metropolitan Life Insurance Co., cannot be read as representing a trend, said Frederic Seltzer, head of the company's statistic bureau. "The mortality rate for most causes of death increased in 1980, but we don't know underlying reasons for that," he said. "We had the influenza outbreak, for example, which affected death from heart and respiratory diseases and the governent has said it might even affect chronic diseases like cancer. "This can be just a one-year fluctuation." For American men who turned 35 last year, the prospect was that they would live and additional 38.2 years while women who turned 35 would live for an additional 44.5 years, Metropolitan said. The company's figures also showed infant mortality cotinuing to decline to an all-time low of 12.5 deaths in the first year of life for every 1,000 babies born. That was down from 13 per 1,000 in 1979 and 20 per 1,000 for the period 1969-71, Metropolitan said. In 1968, the last time the charts fell, life expectancy for all Americans born that year was 70.2 years, down from 70.5 in 1967.