Sept. 10, 2001 -- You feel cranky, icky, just plain yucky -- so of course, it's PMS, right? Many women swear they suffer as their period approaches. Yet a new study shows that for many women, that's a self-fulfilling prophecy. They expect to feel bad, so they do.

"The more a woman believes in the phenomenon of menstrual distress, the more she exaggerates ... the negativity of her symptoms during her last period," writes María Luisa Marván, a psychology researcher at Universidad de las Americas-Puebla. Her study appears in the current issue of the journal Health Psychology.

In her study, Marván questioned 49 women -- all students at a private university in Puebla, Mexico, all white, and all from middle-to-upper class families.

None of the women knew this was a study about PMS, she says. They were "asked if they wanted to be part of a study of lifestyle factors and health," Marván writes.

She questioned each woman numerous times, on the days both before and after menstruation. Among the questions: Did they have cramps, swelling, headache, muscle stiffness, painful breasts, nausea? Did they feel irritable, depressed, anxious, distracted, have trouble concentrating?

Significant numbers of women reported such symptoms during their pre-period days. Yet when asked later about their premenstrual days, many reported much worse symptoms.

Other researchers have noted the impact of popular women's magazines in "bombarding women" with headlines, anecdotes, and studies implying that extreme mood swings are an inevitable part of our menstrual cycle, she writes.

"Many women have a misperception about the meaning of PMS," Marván writes. "Consequently, they amplify their premenstrual changes, reflecting women's cultural stereotypes rather than their actual experience."

Marván's study "confirms what a lot of literature has been showing -- that PMS exists in the minds of women, that it's not clearly an entity," says Alice Domar, PhD, director of the Mind/Body Center for Women's Health in Boston and author of Self-Nurture: Learning to Care for Yourself as Effectively as You Care for Everyone Else.

"The enormous strength of this study is that the women didn't know it was about PMS," Domar tells WebMD. "It shows that this is more of a psychological issue than we previously thought."