Tracking changes in the levels of a certain hormone during pregnancy may help to predict which women may suffer postpartum depression, a small study suggests.

Risk factors include a history of depression, a stressful or anxious pregnancy, a lack of social support, and low self-esteem, but these explain only a portion of why some women develop postpartum depression.

In Monday's online issue of Archives of General Psychiatry, researchers who studied 100 women said they found 12 out of 16 participants who had postpartum depression also had high levels of corticotropin-releasing hormone or CRH.

In the study, high levels of CRH at 25 weeks gestational age helped to predict future postpartum depression or PPD in about three-fourths of women and misclassified 24 per cent, the researchers said.

CRH is normally produced in tiny amounts by the brain's hypothalamus in response to stress.

Hormone spike

During pregnancy, the placenta produces much more of the hormone, which is thought to prepare a woman's body for childbirth. Levels of CRH and other hormones drop after giving birth.

The hormone levels predicted symptoms of postpartum depression at 23 to 26 weeks gestational age, about the same time that levels of the placental type, known as pCRH, surge.

No one knows why pCRH surges, but evidence suggests there may be a link between higher levels of a stress hormone called cortisol early in pregnancy and increased pCRH late in pregnancy, psychologist Ilona Yim of the University of California, Irvine, and her colleagues said.

"Our study has important clinical and theoretical implications," they said. "If our results are replicable, it may be considered useful to implement a pCRH PPD screen into standard prenatal care."

Blood tests are often done at 24 to 28 weeks gestational age to check for gestational diabetes. The postpartum depression screen could be done at the same time, the team suggested.

Learning more about the role of pCRH in postpartum depression could contribute to the development of preventions for the disorder, they added.

Postpartum depression affects up to one in five women four to six weeks after childbirth, and seven per cent of new mothers suffer a major depression.

Learning relaxation techniques at prenatal yoga classes and counselling may help to prevent postpartum depression, and antidepressants may be prescribed to treat it.