Postpartum depression is often blamed on dramatic hormonal changes  inside the bodies of new mothers. Estrogen, progesterone and cortisol all drop dramatically in the hours after childbirth, and some women are thought to be particularly sensitive to this. But a new study suggests an alternate explanation. The brains of women suffering from postpartum depression reacted differently to images of faces that were scared or angry than did the brains of healthy moms.

Researchers from the University of Pittsburgh and the Cardiff University School of Medicine in Britain performed MRI scans on 30 new moms four to 13 weeks after their deliveries. While in the scanner, the women were shown pairs of faces and asked which of them matched a third picture. Get breaking news alerts delivered to your mobile phone. Text BREAKING to 52669. It turned out that compared with the 16 healthy mothers, the 14 mothers who were suffering from postpartum depression showed significantly less activity in the part of the brain called the dorsomedial prefontal cortex, which is involved in emotional responses as well as “the social cognition network that allows an individual to recognize and consider the emotional experiences, values, and goals of others,” according to the study.  That could explain why the depressed mothers were less attuned to the feelings of others, including their babies, the researchers wrote.

The researchers also found that the dorsomedial prefrontal cortex showed less “connectivity” with the left amygdala in the brains of women with postpartum depression. The amygdala plays a central role in processing and storing emotions.

None of the depressed mothers were taking medication for their condition, which would have complicated interpretation of the brain scans. Aside from depression, both groups of mothers had similar demographic, medical and behavioral characteristics, according to the study.

Of course, more research is needed to pinpoint the biological differences in the brains of new mothers that might be responsible for postpartum depression, the researchers wrote.