Life expectancy in the US has dropped for the first time since 1993 — and experts can’t explain why.

Babies born in 2015 should plan to live for 78.8 years, or 0.1 year less than a kid born in 2014, according to a study Thursday from the National Center for Health Statistics.

Men could expect to live 76.3 years, down from 76.5 in 2014. Women’s life expectancy fell from 81.3 years to 81.2.

Researchers had a hard time explaining the downturn.

“This is unusual, and we don’t know what happened,” Jiaquan Xu, the study’s lead author, told the Washington Post.

But in a grim harbinger, the study found that 8 of the 10 traditional leading causes of death increased in 2015.

“With four years, you’re starting to see some indication of something a little more ominous,” warned University of Illinois-Chicago researcher S. Jay Olshansky.

US life expectancy had been inching north for years since the end of World War II, due to advances in medical science and effective public health campaigns.

For example, an American born in 1950 was expected to live just 68 years.

While gains have been made in fighting cancer, researchers said an upturn in heart disease is the likely culprit in putting Americans on a faster road to death.

Thursday’s findings didn’t break down data by region, education or income. But previous studies have pointed to increasing health challenges for less wealthy, white rural Americans.

“The troubling trends are most pronounced for the people who are the most disadvantaged,” said Syracuse University researcher Jennifer Karas Montez, an expert on patterns of death.

“But if we don’t know why life expectancy is decreasing for some groups, we can’t be confident that it won’t start declining for others.”

Women in Japan currently have the longest average lifespan at 86.8 years, while men in Switzerland can anticipate 81.3 years.

Sierra Leone has the lowest life expectancy, with women at 50.8 years and men at 49.3.

With Post wire services