Life expectancy among some women in the U.S. is steadily declining, according to the latest research published in the journal Health Affairs.

The study indicates that in almost half of the country’s counties, women under the age of 75 are dying at rates higher than before.

This is the first study of its kind to identify the trend, which is particularly evident among low-income white women.

Experts believe that the reason women are dying younger in certain parts of the country is due to increased smoking rates, and obesity. Although many are still uncertain as to what the true causes really are.

The average lifespan for a girl born this year is 81 years, compared to 76 among boys. Although over recent years this difference has been decreasing.

The study involved analyzing the mortality rates in a total of 3,141 counties across the country over a period of 10 years. They identified that over the past decade female mortality rates have risen in 42.8 percent of U.S. counties, compared to male mortality rates increasing in only 3.4 percent of counties.

Given the fact that some of the counties are not that heavily populated, the researchers calculated five year averages to try and be as accurate as possible. In addition, they adjusted their results to account for factors such as education and income.

Women dying younger than expected in the whole country – accounting for every county – dropped from 324 to 318 per 100,000. However, in a little under half of the counties it actually increased from 317 to 333 per 100,000.

A previous study conducted by researchers from the Institute for Health Metrics and Evaluation (IHME) found that 80% of counties in the USA have fallen further behind the life expectancy average of the top 10 countries in the world. In addition, the study similarly identified that females are doing worse than males.

Life expectancy appears to be increasing among educated and affluent women, while declining among those who haven’t completed high school.

Counties in the South have the highest smoking rates, and obesity in those areas is widespread – these could be factors that explain why mortality is getting worse in some parts of the country.

Dr. Christopher Murray said: “I think the most likely explanation for why mortality is getting worse is those factors are just stronger in those counties.”

He added: “We shouldn’t jump to the conclusion that more people are getting sicker in these geographic areas than previously.”

A few experts believe the data could indicate a migration of healthy women out of rural areas, relocating to more urban locations of the country, where mortality rates have gone down.

However, Murray does not believe that this is the case, as migration didn’t impact male mortality rates.

Written by Joseph Nordqvist