A new University of Wisconsin-Madison analysis uncovered a startling fact: Women's mortality rates in almost 43 percent of American counties actually got worse during the 15 years ending in 2006.

In other words, while mortality rates improved in most U.S. counties, women in some parts of America, especially those in the West and South, faced worse premature mortality rates (dying at or before age 75, which is considered by epidemiologists to mark premature death).

A mortality rate is a ratio of the number of deaths in a group compared with the total population of the group. The trend showed that female mortality rates rose in 42.8 percent of counties while male mortality rose in 3.4 percent of counties over the same time period.

Higher female mortality was most strongly associated with living in the rural South or the West, higher smoking rates, and lower education rates. The study found that access to primary care physicians did not affect changes in mortality rates.

"We decided to look at the change in health outcomes over time, and were actually shocked to see that female mortality rates were worsening in more than 42 percent of counties," says Dr. David Kindig, professor emeritus of population health sciences at the University of Wisconsin School of Medicine and Public Health.

Read more from the University of Wisconsin.