Pregnancy causes long-term changes to a mother's brain that makes her love her baby. Brain scans of first-time mothers showed that the region of the brain involved in face recognition and emotion 'lit up' brightly in response to her own child's face.
The same response was not seen when mothers were shown pictures of other people' s children.
And first-time fathers did not show the same remarkable response.
But the so-called conventional view of 'baby brain' – forgetfulness and slower responses – was not found in the new mothers, apart from a slight tendency to be worse at learning lists of words.
The changes are so noticeable – and were found to remain two years after giving birth – that just from a brain scan alone, the scientists say they can tell whether a woman is a mother or not.
The changes to the brain involve some shrinkage of the grey matter in a mother' s brain – and are caused by a huge flood of hormones affecting a mother's brain during pregnancy, scientists said.
The results were found in brain scans of 25 first-time mothers before and after giving birth.
Scientists found the regions of the brain that changed most were regions involved in responding to their own baby's face.
When new mothers were shown pictures of their own child, it was the newly-expanded brain region lit up brightly when shown a picture of her own child.
The researchers found that the greater the changes to the brain, the more attached the mother was to their own baby.
As well as a growth in areas of the brain related to bonding with her infant, other areas of the brain exhibit shrinkage.
The authors write: 'This study provides the first glimpses into the extensive changes in brain structure and function that result from first-time pregnancy.
The authors suggest that these changes may prepare a woman for the social demands of imminent motherhood'.
Describing the 'floods' of hormones that course through a woman's body, the authors write levels of the hormone progesterone rise by between 10-15 times that seen before pregnancy, and oestrogen levels 'typically exceed the oestrogen exposure of a woman's entire non-pregnant life.'
The authors say the shrinkage in grey matter represents a 'pruning' or refinement of the brain's nerve network, which may allow it to function more efficiently.
A similar process of thinning of grey matter happens during puberty, according to the research published in Nature Neuroscience.
The area of the brain's cerebral cortex that is most thinned is involved is known as the 'theory of mind network'.
It is suggested that the thinning of the grey matter creates a fine tuning of the brain.
This allows a mother to focus closely on her infant's mental states and processes and this 'has been shown to be important for secure parent-infant attachment' and aids the development of the baby's mental functioning.
The process may help a mother's ability to recognise the needs of her child.
Some new mothers have blamed forgetfulness on 'baby brain' – a foggy state of mind after giving birth.
While the researchers did not find a 'significant' worsening of memory between new mothers and others, there was a 'trend' for the pregnant women to be worst at learning a list of new words.
The authors write that while changes in memory were not 'significant' they said that all the women in the study were highly educated, and 'the observed lack of memory changes may not be generalizable to women of a different educational background'
Elseline Hoekzema from the University of Barcelona and colleagues compared the 25 first-time mothers to the brains of 19 first-time fathers, 17 men without children and 20 women who had never given birth.
They said the pattern of structural changes they found could be used to distinguish the brains of women who had eventually given birth from those who did not, as well as predict the quality of mothers' attachment to their infants after labour.
The authors observed increased neural activity in some of these pregnancy-modified brain regions when they showed mothers pictures of their own infants, relative to images of other babies.
Finally, a follow-up imaging session determined that almost all of these grey-matter reductions were maintained in the first-time mothers nearly two years after giving birth, except for a partial recovery of grey-matter volume in the hippocampus, a region associated with memory.
The researchers observed no differences between mothers who conceived naturally and those who conceived by IVF.
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