The pregnancy-induced ditziness which many women experience is caused by increased activity in the area of the brain related to emotional skills.
Many women say they become more forgetful, oversensitive and less able to focus on logical tasks during pregnancy.
But researchers at the University of London found that during pregnancy, women use the right side of their brain more than new mothers do, as they prepare to bond with their babies.
Dr Victoria Bourne, from the Department of Psychology at Royal Holloway, said: "Our findings give us a significant insight into the 'baby brain' phenomenon that makes a woman more sensitive during the child bearing process.
"The results suggest that during pregnancy, there are changes in how the brain processes facial emotions that ensure that mothers are neurologically prepared to bond with their babies at birth."
Researchers examined the brain activity of 39 pregnant women and new mothers as they looked at images of adult and baby faces making either positive or negative expressions.
The results showed that pregnant women used the right side of their brain more than new mothers, particularly when processing positive emotions.
The study used a faces test which uses images made of one half of a neutral face combined with one half of an emotive face to see which side of the participants' brain is used to process positive and negative emotions.
Dr Bourne said: "We know from previous research that pregnant women and new mothers are more sensitive to emotional expressions, particularly when looking at babies' faces.
"We also know that new mothers who demonstrate symptoms of postnatal depression sometimes interpret their baby's emotional expressions as more negative than they really are.
"Discovering the neuropsychological processes that may underpin these changes is a key step towards understanding how they might influence a mother's bonding with her baby."