For the first time in decades, the life expectancy of the average American has changed — and not for the better.

A new study from researchers presented by the National Center for Health Statistics (NCHS), which was released Thursday, showed a slight decrease in life expectancy in the United States in 2015. “This is unusual, and we don’t know what happened,” said Jiaquan Xu, an epidemiologist and lead author of the study, to The Washington Post. “So many leading causes of death increased.”

So how bad is it?

The good news: life expectancy hasn't been cut down by years, but by months. The U.S. population's life expectancy in 2015 dropped from 78.9 years to 78.8 from the previous year, the study reports.

Researchers also found an increase in death rates from 2014 to 2015 for black men, white women and white men with no significant changes among Hispanic men, Hispanic women and black women. Overall, men saw a slightly more drastic difference, from an average expectancy of 76.5 years to 76.3. Women only dipped from 81.3 years to 81.2. (Once again, women continue to live longer than men.)

So what happened?

There's always the possibility that 2015 was a fluke. Robert Anderson, chief of the mortality statistics branch at the NCHS told NPR, "we'll have to see what happens in the second half of 2016" before we can know definitively. He added that he does believe that the new statistics shouldn't be ignored or dismissed.

"This is a big deal. There's not a better indicator of well-being than life expectancy. The fact that it's leveling off in the U.S. is a striking finding," Dr. Phil Morgan, Sociology Director at the Carolina Population Center with a focus on fertility in the U.S. who wasn't involved in the study, told NPR.

The biggest reason researchers can point to behind this drop in life expectancy is the overall increase in death rates for eight out of the 10 leading causes of death categories with heart diseases taking the top spot. More Americans are dying as the result of heart disease and stroke than before and the obesity epidemic could possibly be the culprit, Rob Stein of NPR notes.

However, as the research shows, more Americans also died in 2015 of suicide and "unintentional injuries" than in 2014.

Among the top 10 "leading causes of death" in 2015 are heart disease, cancer and chronic lower respiratory diseases in the top three spots, followed by unintentional injuries, stroke, Alzheimer’s disease and diabetes. 

Overall, the United States is actually lagging behind other countries when it comes to long life. The World Bank data shows a life expectancy in Italy of 83 years, 84 years in Japan and 82 years in France. C'est la vie.

[H/T NPR]