NEW YORK (Reuters Health) - Women with a history of postpartum depression tend to have unusually high levels of copper in their blood, a new study has found -- suggesting the mineral may play some role in the disorder.

While many women go through a short spell of the "baby blues" after giving birth, about 15 percent suffer full-blown postpartum depression. It's not clear why some women are more vulnerable than others.

The new findings suggest that the body's regulation of copper levels may be involved, researchers report in the Journal of Trace Elements in Medicine and Biology.

They found that blood copper levels were significantly higher among 78 women with a history of postpartum depression compared with non-depressed women and those who'd suffered depression unrelated to childbirth.

"This could very well be the missing link in the mystery of postpartum depression," said Dr. William J. Walsh, a co-author of the study and director of research at the Pfeiffer Treatment Center and the Health Research Institute in Warrenville, Illinois.

The center focuses on treating various mood and behavioral disorders by finding and treating "biochemical imbalances" they believe are at the roots of the problem.

In the case of postpartum depression, Walsh explained in an interview, the theory is that some women have a problem in the body's built-in system for clearing excess copper.

During pregnancy, Walsh said, a woman's copper levels normally go up, more than doubling, before normalizing after childbirth. But he and his colleagues speculate that in women who develop postpartum depression, copper levels do not normalize for some reason -- most likely, they suspect, because of a genetically determined flaw in the protein that regulates copper levels.

Persistently high copper levels might contribute to postpartum depression because of the metal's role in brain chemistry, according to Walsh. Excess copper in the brain, he said, can alter the balance of dopamine and norepinephrine, two mood-regulating chemicals.

For their study, Walsh and colleague Dr. John W. Crayton examined data on a group of patients seen at the Pfeiffer center between 1990 and 2002. They focused on blood copper levels among 78 women with a history of postpartum depression, 148 with a history of depression but no postpartum symptoms and 28 with no history of depression.

The average copper level for the postpartum group was 131 micrograms per deciliter of blood, versus 111 mcg/dL in the depression group and 106 mcg/dL among non-depressed women.

These differences do not prove that high copper levels caused the women's postpartum depression, Walsh acknowledged.

But he said that in his clinic's experience, women with postpartum depression have responded to therapy with supplements that "normalize" their blood copper levels.

However, there have been no rigorous, controlled clinical trials to prove such treatment works. That, Walsh said, is the next step in the research.