Don't beat yourself up about baby blues; your hormones are not to blame. A new study has found that the brains of women with post-natal depression process negative emotions differently to new mothers without the condition.
Mary Phillips at the University of Pittsburgh, Pennsylvania, and colleagues, used functional MRI scans to compare the brains of 14 women with post-natal depression and 16 new mothers without while they looked at pictures of angry and sad faces.
They found that the depressed women had less activity in the dorsomedial prefrontal cortex - an area of the brain thought to pick up on emotional cues and mediate emotional responses - than the non-depressed women.
Women with post-natal depression tend to find it hard to bond with their babies, reports New Scientist.
Phillips suggests that this might be linked to a more general loss of interest in social interactions.
It was previously found that people with depression tend to be more sensitive to negative images than people free of it.
The new study has shown that new mothers with post-natal depression seem to buck this trend - shutting out negativity and not reacting to it.
The apparent deficit in brain activity was not seen in the healthy new mothers, she says, so it is unlikely that pregnancy alone causes the effect.
But more research is needed to find our whether the deficit is caused by depression or if women with less activity are predisposed to post-natal depression.
The study has been published in The American Journal of Psychiatry.