The popular belief that women's minds turn to mush during pregnancy and birth is completely wrong and their grey matter actually increases, they say.
Research published by the American Psychological Association found that the brains of new mothers bulked up as they coped with the steep learning curve of dealing with a newborn.
Mothers who gushed the most about their babies showed the greatest growth in key parts of the brain, it was found.
The researchers at the National Institute of Mental Health in Maryland scanned the brains of 19 women who gave birth to 10 to boys and nine to girls.
A comparison of images taken two to four weeks and three to four months after the women gave birth showed that grey matter volume increased by a small but significant amount in various parts of the brain.
In adults, grey matter does not ordinarily change over a few months without significant learning, brain injury or illness, or major environmental change.
The authors speculated that hormone levels and the need to cope with the challenges of a baby led to the increase in brain cells.
The areas affected are involved with motivation – the hypothalamus -, reward and emotion processing – the amygdala – senses – parietal lobe – and reasoning and judgment – the prefrontal cortex.
The findings were published in the journal Behavioural Neuroscience.
The motivation to take care of a baby, and the hallmark traits of motherhood, might be less of an instinctive response and more of a result of active brain building, the neuroscientists Dr Craig Kinsley and Dr Elizabeth Meyer speculated.
Mothers who most enthusiastically rated their babies as special, beautiful, ideal, perfect and so on were significantly more likely to develop bigger brains than the less awestruck mothers in key areas linked to maternal motivation, rewards and the regulation of emotions.
Although these early findings require replication with a larger and more representative sample, they raise intriguing questions about the interaction between mother and child, said the report.