Living to 100 years old is a major milestone that very few people will ever hit. For most of the 20th century, the expected lifespan of both men and women has been rising, leading many to have reasonable hope that becoming a centenarian would be commonplace by the time they got old.

A new study published in the Proceedings of the National Academy of Sciences, however, says that this is unlikely due to a dramatic slowdown in the increase of longevity.

The study used six separate forecasting methods to estimate the life expectancy rates of people living in 23 high-income, low-mortality countries. What they found is that those born from 1939 to 2000 (the range of the study) are very unlikely to hit that 100th birthday.

This may not come as a surprise to most people since the modern lifestyles of people have become very unhealthy. More people are overweight and living a very sedentary lifestyle, both of which are known to cause many health issues over time.

For much of the 20th century, life expectancy was going up extremely quickly, but that is not the case anymore. The reason for the slowdown, however, is not the unhealthy lifestyles. It is actually because medical advancements have already made such amazing improvements in many areas that it is hard to keep making progress at the same rate.

From 1900 to 1938, the average lifespan was going up by .46 years per year. The bulk of that increase, however, was from a dramatic reduction in infant mortality and the deaths of young people due to diseases that today are very treatable.

In order to continue to extend the lifespan of humans at the same rate, the medical industry would need to start having dramatic advancements in its ability to cure diseases and conditions that are common much later in life, which is naturally more difficult. Hector Pifarre i Arolas is the study author, and in a statement, he said:

“In the absence of any major breakthroughs that significantly extend human life, life expectancy would still not match the rapid increases seen in the early 20th century even if adult survival improved twice as fast as we predict.”

Finding cures to cancer, heart disease, and other common causes of death in the elderly is difficult. Of course, a single medical breakthrough could immediately reverse these trends and find the average lifespans growing very rapidly once again.

If you found that story interesting, learn more about why people often wake up around 3 AM and keep doing it for life.