The study measures the relationship between depression and a hormone
Measuring levels of a hormone midway through pregnancy may predict a woman's risk of postnatal depression, say US researchers.
In a study of 100 women, levels of the pCRH hormone at 25 weeks helped predict three-quarters of those who developed the "baby blues".
The researchers said, if proven in larger studies, the test could be used routinely to screen for depression.
The findings are published in Archives of General Psychiatry.
Postnatal depression generally starts within four to six weeks of giving birth and affects 10-15% of mothers..
Known risk factors include a history of depression, stressful life events, a lack of social support, low self-esteem, anxiety or stress during pregnancy.
Other studies have also looked at the effects of hormones during pregnancy.
But this is the first study to specifically measure the relationship with depression and pCRH (placental corticotropin-releasing hormone).
Blood samples were taken from pregnant women at two medical centres in California at 15, 19, 25, 31 and 37 weeks' of pregnancy and symptoms of depression were assessed at the last four pregnancy visits as well as at an average of 8.7 weeks after giving birth.
In all, 16 women developed symptoms of postnatal depression.
The researchers found that levels of pCRH when the women were 25 weeks pregnant strongly predicted future development of depression.
It was more accurate when depressive symptoms in the middle of pregnancy were taken into account.
Levels of the hormone increase during pregnancy which increases the production of cortisol - which helps the body deal with stress.
After birth, levels of pCRH drop dramatically leading to a dip in cortisol.
The theory is that women who have have higher levels of pCRH this dip is more pronounced.
Writing in the journal, Dr Ilona Yim, an assistant professor at the University of California, said: "If our results are replicable, it may be considered useful to implement a pCRH postpartum depression screen into standard prenatal care.
"Because blood draws to screen for gestational diabetes are typically performed at 24 to 28 weeks' gestational age, a potential postpartum depression screen could be completed at the same time."
Dr Patrick O'Brien, spokesman for the Royal College of Obstetricians and Gynaecologists, said the results were very interesting but needed to be tested on a bigger sample group.
He said: "The thing with postpartum depression is that once it is identified it usually can be treated very well.
"But the problem is that often women withdraw within themselves and don't seek help.
"If the test was proved to work, then you could increase surveillance of those at risk."