Overall, women outlive men, as they have for generations. The latest numbers show the average life span for a baby girl born today is 81, and for a baby boy, it's 76.
But the gap has been narrowing, and federal data show the phenomenon of some women losing ground appears to have begun in the late 1980s. Studies have begun to spotlight it only in the last few years.
Researchers don't know how many women are affected, but Jennifer Karas Montez, a Harvard School of Public Health sociologist, said a good estimate is 12 percent.
The new study, released Monday by the journal Health Affairs, found declining female life expectancy in 43 percent of the nation's 3,141 counties.
Researchers David Kindig and Erika Cheng of the University of Wisconsin used federal death data over 10 years to calculate mortality rates for women under age 76. They also used statistical methods to control for factors like income and education.
Nationwide, the rate of women dying younger than would be expected fell from 324 to 318 per 100,000. But in 1,344 counties, the average premature death rate rose from 317 to 333 per 100,000.
Some other studies have found declines in life expectancy for white women who never earned a high school diploma. Meanwhile, life spans seem to be growing for more educated and affluent women.
The studies spotlight regional differences. Some of the highest smoking rates are in Southern states, where the proportion of women who failed to finish high school is also highest.