When women produced a "surge" of the hormone between 23 and 26 weeks into pregnancy this was strongly linked with later post-natal or "postpartum" depression.
US research showed that the condition, which usually occurs between four and six weeks after giving birth, is different from the period of tearfulness known as the "baby blues".
Post-natal depression affects roughly one in 500 mothers and in some cases symptoms are so severe that women have been driven to suicide or infanticide.
More than 17,000 women in the UK may suffer undetected post-natal depression each year, according to research.
Scientists in California looked at 100 pregnant women and tested the levels of placental corticotropin-releasing hormone (pCRH) to see if it played a role.
Of those women, 16 went on to develop post-natal depression symptoms and when their pCRH levels were examined it was found that within a narrow band of pregnancy the hormone gave a good indication of risk.
Related article on NursingTimes.net: Supporting vulnerable women during pregnancy