Women with longer fertile lifespans have a lower risk of contracting Parkinson's disease, according to medical research in the United States published Wednesday.

"These findings suggest that longer duration of exposure to the body's own (endogenous) hormones may help protect the brain cells that are affected by Parkinson's disease," said the authors of the study.

A woman's fertile lifespan stretches from her first menstruation to menopause. Women with fertile lifespans longer than 39 years had 25 percent lower risk of developing Parkinson's, compared to women with ones 33 years or shorter, said the researchers, who will presented their findings at the annual meeting of the American Academy of Neurology, April 25-May 2 in Seattle, Washington.

They also found that women who had four or more pregnancies were 20 percent more at risk for Parkinson's than those with three or fewer pregnancies.

And women who underwent hysterectomy, or surgical menopause, had almost twice the risk of developing Parkinson's. The risk was double if they had previously taken hormone therapy and stopped than if they had never taken hormone therapy.

Taking hormones did not have any effect on Parkinson's risk for women who had natural menopause, the researchers said.

"This study does not support a role for treatment with hormone therapy in Parkinson's, but there are still many unanswered questions," said study author Rachel Saunders-Pullman, of Albert Einstein College of Medicine in Bronx, New York.