A simple blood test could help identify women who are most likely to suffer from postnatal depression in advance.
PND affects one in seven women and is far more severe than 'baby blues. Mothers may struggle to bond with their babies and treatment and support are vital.
Researchers at Warwick Medical School found women who developed the condition were more likely to have variants of two receptor genes involved in the body's stress response, the Daily Mail reported.
The discovery could lead to earlier treatment for women who are likely to suffer from postnatal depression.
At present women are often diagnosed by chance if a midwife or relative notices the symptoms, which usually appear a fortnight after birth. However, many new mothers are unwilling to admit they are suffering in the mistaken belief the condition means they are a 'bad mother.'
Presenting the teams research to the International Congress of Endocrinology, Professor Dimitris Grammatopoulos said: "Current screening policies rely on the opportunistic finding of PND cases using screening tools such as the Edinburgh Postnatal Depression Score (EPDS), but such tests cannot identify women at risk, ahead of them developing the condition."
The scientists assessed a group of 200 pregnant women for PND using the EPDS, once during their first visit to the ante-natal clinic, and again two to eight weeks after they had given birth.
They found that the women who developed PND were more likely to have a DNA sequence variation in two receptor genes (the glucocorticoid receptor and the corticotrophin-releasing hormone receptor-1).
These receptors control the activity of the hypothalamo-pituitary adrenal (HPA) axis, which control the activity of hormones triggered in response to stress.
The finding appears to show that postnatal depression is a specific subgroup of depression with a distinct genetic element, which means that some women are genetically more reactive to the environmental factors that trigger depression.
Professor Grammatopoulos said their study was the first to show a link between the functioning of the HPA axis and postnatal depression.
"We think that we have made an important step forward in characterizing the prospective risks and are therefore paving the way for timely, appropriate medical treatment for women who are likely to develop PND," he said.
The team now intend to conduct further research on other genetic variants of the HPA axis in a larger, multi-centre study involving women from Coventry, Birmingham, and London.
PND is a serious condition and quite different from the 'baby blues', which is milder and shorter-lived. Symptoms include sadness, changes in eating and sleeping patterns, crying episodes, reduced libido, anxiety and irritability.
Effects on children can be significant; for example, depressed mothers are less likely to be affectionate towards and to play with their children and they may use less 'baby talk', which is designed to engage the child's attention. This may lead to learning and emotional difficulties for the children in later life.
Although it may seem evident that PND is caused by some kind of hormonal upheaval the role of the HPA axis in this form of depression has not been proved until now.
"We believe that we have made a discovery with important clinical and social implications. If we can identify women likely to suffer from PND in advance so that they can be treated appropriately and at an early stage, we will have improved the lives not just of the parents, but also of their children," Professor Grammatopoulos concluded.