Between 1969 and 2010, the overall death rate in the US for all causes of death combined has shown a marked and continued decrease, a new analysis by the American Cancer Society has found.

But the findings also include one puzzling trend that could be cause for concern: for the most recent years in the analysis, 2010–2013, the rate of decline in the death rate slowed until it was virtually flat, suggesting that the advancements made in health and longevity in the preceding four decades may have now plateaued.

The decline in death rates for the entire period from 1969 through to 2013 shows an overwhelmingly positive development on the whole – with death rates dropping by 43 percent since 1969, from 1,279 per 100,000 people to 730 per 100,000.

This drop, amounting to an average annual decrease of 1.3 percent, is largely due to advancements in health and medical care that have seen declines in the standalone death rates for five of the six leading causes of death in the US.

For example, the rate of death for stroke per 100,000 people declined by 77 percent; heart disease by 68 percent; unintentional injuries by 40 percent; cancer by 18 percent; and diabetes by 17 percent. Chronic obstructive pulmonary disease (COPD) was the sole exception here, seeing a 101 percent increase in the period.

The reasons for the decreases are varying, including things like better tobacco controls, advancements in early cancer detection, more effective cancer treatments, and better motor vehicle safety.

What's concerning about the statistics is that the decline in the death rate at the very end of the period – between 2010 to 2013 – is so slight, it isn't even statistically significant. In other words, unless this is just a temporary pause, it looks like advancements made in health – at least to the extent that they're measurable by the death rate – have come to an end, and we could even see a swing back in the other direction.

"I was surprised," Ahmedin Jemal of the American Cancer Society and leader of the study, told Sabrina Tavernise at The New York Times. "We were expecting to see more declines."

While the researchers aren't necessarily calling this short-term swing in the data the start of a definite trend, it's certainly remarkable, and the researchers suggest it might be signalling a delayed effect of the obesity epidemic that has affected the US since the 1980s.

Others disagree. David Cutler, an economics professor at Harvard University, told Tavernise that the big gains from various life-saving medicines are no longer making a positive impact on longevity, with saturation of modern treatments explaining why the death rate has effectively flattened. "At some point, everybody is taking a statin [for cholesterol] and you top out," he said.

Another explanation could be that death rates are a useful measure for tracking mortality levels within the population, but not necessarily for actually tracking the health of the population, which is related, but also different.

"The medical community seems to be under a fog that we can constantly and forever reduce death rates, and that's simply not true," said S. Jay Olshansky, a public health professor from the University of Illinois at Chicago. "You need to look at the health status of the living."

The findings are published in The Journal of the American Medical Association.