AVERAGE AMERICAN LIVING LONGER.

Byline: Daily News Staff and Wire Services

U.S. infant mortality rates reached their lowest level in history last year and Americans' average life expectancy rose to an all-time high of 76-1/2 years, the latest government statistics tracking the nation's health show.

The infant mortality rate in Los Angeles County was 5.9 deaths per 1,000 births in 1997, according to the county Department of Health.

And life expectancy in the city of Los Angeles was 78 years, five months, the most recent available figures show.

Teen pregnancies nationwide, as measured by the number of live births, fell by 3 percent, continuing a downward trend that started six years ago, according to data compiled by a research arm of the Centers for Disease Control and Prevention and published today in the journal Pediatrics.

``All of the trends are positive,'' said Dr. Bernard Guyer of the Johns Hopkins School of Hygiene and Public Health, who analyzed the government data in Pediatrics. ``They are all going in the right direction.''

The 1997 infant mortality rate - a measure of the number of deaths before a baby turns 1 - was 7.1 deaths per 1,000 births. That was about 3 percent lower than the 1996 rate of 7.3 deaths per 1,000 live births.

The drop in infant deaths came despite a 7.5 percent increase in the number of low-birth-weight babies, and an increase of about 2 percent in the number of births by women late in their childbearing years. Pregnancy and birth complications are more common among older women.

There also was a sharp rise in multiple births, another medical complication that affects infant mortality. Guyer said the number of twins rose by 5 percent, while the number of ``higher order multiple births'' - triplets or more - increased by 20 percent.

That increase has come about because more women are using fertility drugs.

Medical science is combating such potential complications by developing improved ways to treat and care for underweight and premature babies, Guyer said.

``We have become very good at treating them regardless of their birth weight,'' he said.

Yet despite the improvement in infant death rates, ``we still rank pretty poorly'' compared to other industrialized countries, said Guyer.

Singapore, for instance, had an infant mortality rate of 3.8 per 1,000 live births in 1996, a little over half the infant death rate in the United States.

The number of years a person born in the United States can expect to live increased, in part, because of falling death rates from major diseases and from accidents, homicides and suicides. Deaths from HIV, or AIDS, for instance, dropped by 47 percent in 1997.

A longer life expectancy, said Guyer, ``reflects improvement at both ends of the life spectrum. The fact that infant mortality is down means that babies born this year are going to have a longer life expectancy. At the other end, we are able to prolong life longer and the aged are healthier.''

The most dramatic improvement was among African-American males, whose life expectancy increased by 1.2 years to 67.3. For African-American females, it improved to 74.7 years, an increase of half a year.

For white females, life expectancy rose to 79.3 years, up one-tenth of a year. And for white males, it is now 74.3, an improvement of four-tenths of a year.

Among other findings:

Births to teen-age mothers fell in 1997 for the sixth consecutive year to 52.9 births per 1,000 females ages 15 to 19. That is 3 percent lower than the 1996 rate of 54.4 and 15 percent lower than the 1991 rate of 62.1. ``All of the stuff from the last decade - safer sex, more access to contraception - appears to have had an effect,'' said Guyer.

Based on 1996 statistics, the latest available, Maine had the lowest infant mortality rate at 4.4 per 1,000 live births. The highest rate was in the District of Columbia, with 14.9 infant deaths per 1,000 live births.

The number of very low birth-weight babies, those born at less than 3.3 pounds, increased slightly, to 1.41 percent of all live births in 1997, from 1.37 percent in 1996.

Low birth-weight babies, those weighing less than 5.5 pounds at birth, increased to 7.5 percent in 1997 from 7.4 percent in 1996, continuing a steady increase. The rate in 1984 was 6.7 percent of all live births.