Image copyright SPL Image caption Deaths from dementia saw one of the biggest rises

Life expectancy in the United States has declined for the first time in more than two decades.

Data from the National Center for Health Statistics showed a drop for men from 76.5 years in 2014 to 76.3 in 2015, and from 81.3 to 81.2 for women.

The preliminary figures show rises in several causes of death, especially heart disease, dementia and accidental infant deaths.

Life expectancy last fell during the peak of the HIV/Aids crisis in 1993.

It has improved slightly but steadily in most of the years since World War Two, rising from a little more than 68 years in 1950.

It also fell in 1980, after a severe outbreak of flu.

Overall life expectancy for men and women is now 78.8 years, a decrease of 0.1 year from 2014.

"This is unusual," lead author Jiaquan Xu, an epidemiologist at the NCHS, told AFP news agency. "2015 is kind of different from every year. It looks like much more death than we have seen in the last few years."

The report is based mainly on 2015 death certificates.

Image copyright Getty Images Image caption Rising obesity, heart disease and diabetes are all concerns

How far has life expectancy declined?

A decline of 0.1 years in life expectancy means people are dying, on average, a little over a month earlier - or two months earlier for men.

To compare it with the two other declines in the past 30 years, the drop from 1992 to 1993 was 0.3 years, and the drop from 1979 to 1980 was 0.2%.

What's also worrying some experts is that the trend had been largely flat for the preceding three years, rather than steady increase which has prevailed since the 1970s.

What's causing the drop?

The figures show a mixture of factors. Death rates have risen for eight out of 10 of the leading causes of death: heart disease (0.9% rise), chronic lower respiratory diseases (2.7% rise), unintentional injuries (6.7% rise), stroke (3% rise), Alzheimer's disease (15.7% rise), diabetes (1.9% rise), kidney disease (1.5% rise) and suicide (2.3% rise).

Heart disease is the biggest killer - accounting for more than four times as many deaths as each of the others - so even the relatively small 0.9% rise in the heart disease death rate is a major contributor.

Two of the biggest rises were deaths from Alzheimer's disease and also an 11.3% increase in the rate of death for babies under one due to unintentional injuries.

Experts point to obesity levels, an ageing population and economic struggles as wider factors.

What's behind the rise in accidental infant deaths?

"Most of them died from accidental suffocation and strangulation in bed," said Jiaquan Xu.

Michael Grosso, medical director at Northwell Health's Huntington Hospital in New York, told AFP that these deaths would include car crashes, falls, suffocation and fires, and were therefore complex to explain.

He linked the rise to "social stressors", such as financial pressures and addiction.

"The dramatic upswing in the use of opiates and narcotic use across our country is potentially a big factor in driving a phenomenon like accidental injury," he said.

The Centers for Disease Control and Prevention says the country is "in midst of an opioid overdose epidemic", with a record 28,000 people killed in 2014. No figures are yet available for 2015, though the 6.7% rise in deaths caused by "unintentional injuries" may be partly related.

Image copyright AP Image caption A decline in the cancer death rate is one of the few positive trends

Is there any good news?

The death rate for cancer has gone down 1.7%, which is significant as cancer is the second-biggest cause of death, causing almost as many fatalities as heart disease.

But it seems that fast-developing research into cancer treatments, as well as campaigns on public education and early detection, are having an impact.

How does the US compare with other countries?

The US ranks 28th out of 43 OECD countries, according to 2014 figures - the most recent available. It is just behind the Czech Republic, Chile and Costa Rica, and just above Turkey, Poland and Estonia.

The world's highest life expectancy is in Japan, which is well known for the longevity of its elderly citizens. People there live, on average, to 83.7 years, which is followed by Switzerland and Spain on 83.3.

The world's lowest life expectancy is in Sierra Leone, at 50.1 years, according to the World Health Organization.