In a study of 100 women, researchers at the University of California, Irvine, found 12 out of 16 women who had postpartum depression also had high levels of a hormone circulating in the placenta midway through pregnancy.

Corticotropin-releasing hormone, or CRH, is normally produced in tiny amounts by the hypothalamus near the brain in response to stress.

In pregnant women, the placenta pumps out 100 times more CRH than is normally produced by the hypothalamus. The hormone has been nicknamed the "placental clock" because it is thought to prepare the woman's body for childbirth, said psychologist Ilona Yim, who worked on the study.

Levels of CRH and other hormones drop after the mother gives birth, which Yim said causes hormone "withdrawal" that can create havoc with the endocrine system.

"It puts the whole system out of whack," she said in a telephone interview.

CRH triggers a cascade of reactions in the pituitary and adrenal glands that culminates in increased output of stress hormones like cortisol.

Previous research suggested an overactive stress response plays a role in heart disease, Alzheimer's disease and autoimmune disorders. Stress hormones produced by a dysfunctional endocrine system may also trigger mental disorders like depression.

"When they look at the brains of suicide victims, they have elevated levels" of these stress hormones, Yim said.

Postpartum depression strikes those who experience the biggest change in the hormone levels, Yim found. Women who had high levels of CRH 25 weeks into their pregnancy were more likely to experience postpartum depression.

"This is the first study that implicates CRH in postpartum depression. That has implications for understanding this disorder," she said, adding the results needed to be replicated on a larger scale.

Postpartum depression affects as many as 1 in 5 women four to six weeks after childbirth, and 7 per cent of new mothers suffer a major depression. If not addressed, women can become so despondent they attempt suicide, and some harm or neglect their newborns.

Previous bouts of depression, a lack of social support, low self-esteem and a stressful pregnancy all increase the likelihood of postpartum depression, according to the study, which appeared in the Archives of General Psychiatry.

The report suggested that a routine blood screening, which would coincide with a commonly performed prenatal diabetes test, could determine levels of the hormone at around 25 weeks to identify women at risk.