ATLANTA (AP) – U.S. life expectancy has risen to a new high, now standing at nearly 78 years, the government reported Wednesday.

The increase is due mainly to falling death rates in almost all the leading causes of death. The average life expectancy for babies born in 2007 is nearly three months greater than for children born in 2006.

The new U.S. data is a preliminary report based on about 90 percent of the death certificates collected in 2007. It comes from the National Center for Health Statistics, part of the Centers for Disease Control and Prevention.

Life expectancy is the period a child born in 2007 is expected to live, assuming mortality trends stay constant. U.S. life expectancy has grown nearly one and a half years in the past decade, and is now at an all-time-high.

Last year, the CDC said U.S. life expectancy had inched above 78 years. But the CDC recently changed how it calculates life expectancy, which caused a small shrink in estimates to below 78.

The United States continues to lag behind about 30 other countries in estimated life span. Japan has the longest life expectancy – 83 years for children born in 2007, according to the World Health Organization.

The CDC report found that the number of deaths and the overall death rate dropped from 2006 – to about 760 deaths per 100,000 people from about 776. The death rate has been falling for eight straight years, and is half of what it was 60 years ago.

Heart disease and cancer together are the cause of nearly half of U.S. fatalities. The death rate from heart disease dropped nearly 5 percent in 2007, and the cancer death rate fell nearly 2 percent, according to the report.

The HIV death rate dropped 10 percent, the biggest one-year decline in 10 years.

“It was kind of a surprise to see it go down so much” and it’s unclear if it will be a one-year fluke or not, said Bob Anderson, chief of the agency’s mortality statistics branch.

The diabetes death rate fell about 4 percent, allowing Alzheimer’s disease to surpass diabetes to become the sixth leading cause of death. Alzheimer’s has been climbing the death chart in recent years, though that may be partly because declines in other causes are enabling more people to live long enough to die from Alzheimer’s, Anderson said.

The nation’s infant mortality rate rose slightly in 2007, to 6.77 infant deaths per 1,000 births, but the rise was not statistically significant. It has been at about the same level for several years.

That’s not a shock, some experts said. Medical care improvements can improve infant survival, but they also mean that some troubled pregnancies now make it to infancy before death, said Paul Terry, an assistant professor of epidemiology at Atlanta’s Emory University.

Another recent CDC report containing early data for 2008 counted 2.45 million deaths last year. That’s an increase of more than 29,000 deaths from the 2.42 million deaths in 2007.

CDC data sometimes changes as more records come in and researchers eliminate duplicate reports. But it’s likely an increase will hold up because of the growing number of elderly, experts said.