Women in the military who experience combat have a much greater risk than those who don't of developing post-traumatic stress disorder (PTSD) and other mental health issues, a U.S. study suggests.

Compared to their peers without any combat exposure, enlisted women who had just one combat experience were over four times more likely to screen positive for PTSD in post-deployment exams, the study found.

With three or more combat experiences, the PTSD risk was more than 20 times greater.

"The findings we have are quite similar to the findings of previous studies with largely male samples, which found that men who experienced combat exposure were also more likely to suffer from mental health problems when returning home," said lead study author Rachel Sayko Adams of Brandeis University in Waltham, Massachusetts.

While the study didn't compare women to men, some of the results suggest the need for more research into gender differences in combat experiences and post-deployment mental health problems, Adams added by email.

That's because a substantial number of women reported being injured, wounded, assaulted or hurt during deployment. About 17 percent of enlisted women on active duty reported this, as did 29 percent of women in the National Guard or reserves.

"It is unknown if this item captured instances of being victim to sexual assault while deployed, which is more common among women military members, and may also have a unique negative impact on mental health problems once back at home," Adams said.

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Researchers examined data from post-deployment mental health screenings for more than 42,000 women enlisted in the U.S. Army and deployed in Iraq and Afghanistan from 2008 to 2011.

Although women were formally excluded from direct combat roles until 2013, they still served in war zones. During the operations in Iraq and Afghanistan, women made up about 11 percent of deployments, Adams and colleagues note in the Journal of Traumatic Stress.

To assess combat exposure, researchers looked at women's responses to screening questions that asked, among other things, whether they were wounded or "or otherwise hurt," saw people killed or wounded, fought in direct combat or fired a weapon, or experienced feeling "in great danger of being killed."

With one combat exposure, women were almost twice as likely to screen positive for depression and 38 percent more likely to screen positive for at-risk drinking, the study found.

When women had at least three combat exposures, the odds of screening positive for depression was almost four times greater, while the odds of at-risk drinking was 79 percent higher.

One limitation of the study is that it relied on initial post-deployment screenings, which flag risk factors that merit follow-up exams but are not a formal diagnosis of PTSD or other mental health problems, the authors note.

Even so, the results are similar to findings from previous studies of male noncombatants who are exposed to combat-related stressors, said Dr. Alan Peterson, a psychologist at the University of Texas Health Science Center in San Antonio who specializes in combat-related PTSD.

It's possible that service members may be more resilient when exposed to combat if they have been trained and prepared to expect this, Peterson, who wasn't involved in the current study, said by email. Men and women are both more vulnerable to PTSD when combat situations take them by surprise.

"However, most female service members are not trained to serve in combat roles; invariably, many get exposed to significant, unexpected combat-related traumatic events," Peterson said. "Therefore, female service members are at significant risk for combat-related PTSD during deployments."