U.S. life expectancy has dropped slightly - by about a month - after mostly inching up for many years, the government reported Thursday.

The preliminary report indicates that a baby born in 2008 can expect to live to 77.8 years if current trends continue. That's down a bit from an all-time high of 77.9 years for 2007. A similar dip occurred in 2005, and life expectancy also dropped in 1993.

The lead author of the report, Arialdi Minino, called the 2008 change minuscule and said it would take years of data to determine if that's a trend.

Life expectancy was down for both men and women. The gap between blacks and whites closed a little, to a 4.6-year difference in life expectancy; black men for the first time topped 70 years. Overall, women continue to live longer, until about 80, compared to 75 for men.

What's behind the slip in overall life expectancy isn't known.

"It's something to keep our eyes on," said Ken Thorpe, a health policy professor at Emory University in Atlanta. He suggested it could be related to rising obesity rates.

The report was released Thursday by the National Center for Health Statistics, which is part of the Centers for Disease Control and Prevention in Atlanta. It's based on nearly all the death certificates for that year; a final report will be issued later.

"2008 was not much different from 2007," said Minino. "Once you look under the hood, and look at the trends and the causes, you do find differences. But overall, it wasn't that different."

Life expectancy figures for Latinos have not been included in the annual reports because of unreliable data. In October, the CDC released its first-ever calculation for Latinos, which showed that those born in 2006 could expect to outlive whites by more than two years and blacks by more than seven.

In other highlights from the 2008 report:

-- Stroke fell from the No. 3 leading cause of death for the first time in five decades. It was surpassed by chronic lower respiratory diseases, which include asthma, emphysema and chronic bronchitis. While the death rate from stroke dropped by 4 percent, the swap in position may be due in part to changes in the definition of the respiratory disease category, which increased 8 percent.

-- The age-adjusted death rate fell for the ninth year in a row, to a low of about 759 deaths per 100,000 people. The number of deaths increased by more than 49,300 to about 2.5 million deaths in 2008.

-- Death rates declined for six of the 15 leading causes of death: heart disease, cancer, stroke, diabetes, homicide and accidents. In addition to chronic lower respiratory diseases, death rates went up for Alzheimer's disease, flu and pneumonia, high blood pressure, suicide and kidney disease.

-- Heart disease and cancer continue to be the two top killers, accounting for about half of all deaths.

-- The infant mortality rate, which has been at about the same level for years, dropped about 2 percent to a record low of 6.59 deaths per 1,000 births. The rate for black infants is about twice that of whites. Birth defects, prematurity and low birth weight are the leading causes.

"Infant mortality is going in the right direction, although the differential between blacks and whites is still too high," said Thorpe.

The president of the American Heart Association, Dr. Ralph Sacco, said the group is "heartened to see a continuing decrease in stroke mortality."