During pregnancy there are changes in how the brain processes positive facial emotions.
This is the finding of a study by Dr Victoria Bourne from Royal Holloway, University of London presented to the British Psychological Society annual conference.
Dr Bourne said: "Pregnant women and new mothers have been found to be more sensitive to emotional expressions particularly when looking at babies' faces. This study examined the neuropsychological mechanisms that may underpin these effects."
Some 39 pregnant women and new mothers looked at images of adult and baby faces with either positive or negative expressions. The images used were made of one half of a neutral face; combined with one half of an emotive face (chimeric faces test). This test is used to see which side of the brain is used to process positive and negative emotions.
The results showed that pregnant women used the right side of their brain more than new mothers, particularly when processing positive emotional expressions.
Dr Bourne continued: "Changes in how the brain processes emotional expressions take place during pregnancy. This is why differences were found between pregnant women and new mothers.
"We know from previous research that new mothers who demonstrate symptoms of post-natal depression sometimes interpret their baby's emotional expressions as more negative than they really are. However, future research needs to focus on the underlying causes of these changes."