European researchers said the scale of brain changes during pregnancy were akin to those seen during adolescence.
But they found no evidence of women`s memory deteriorating.
Many women have said they feel forgetful and emotional during pregnancy and put it down to "pregnancy" or "baby" brain - and, it seems, with good reason.
Hormone surge
Pregnancy is characterised by extreme surges of sex hormones and involves drastic physiological and physical changes in the body, the researchers say.
During those nine months, women experience a flood of oestrogen which is greater than for the whole of the rest of their lives.
Yet research on the effects of pregnancy on the human brain is scarce.
This study, from researchers at the Universitat Autonoma de Barcelona and Leiden University and published in Nature Neuroscience, looked at the brain scans of women before they became pregnant, soon after they gave birth, and two years later, to see how the brain changed.
And they compared these women`s brains with those of 19 first-time fathers, 17 men without children and 20 women who had never given birth.
The researchers found "substantial" reductions in the volume of grey matter in the brains of first-time mothers.
The grey matter changes occurred in areas of the brain involved in social interactions used for attributing thoughts and feelings to other people - known as "theory-of-mind" tasks.
The researchers thought this would give new mothers an advantage in various ways - help them recognise the needs of their child, be more aware of potential social threats and become more attached to their baby.
Just by analysing the brain images, computers were able to pick out the women who had been pregnant.
Bonding with baby
In one task, women were shown pictures of their own babies and other babies and their brain activity was monitored.
The parts of the brain which lit up when they saw pictures of their own babies closely matched the areas where grey matter had been reduced or "fine-tuned" during pregnancy.
The same areas did not light up when pictures of other babies were viewed.
Elseline Hoekzema, study author and postdoctoral researcher at the Institute of Psychology at Leiden University in the Netherlands, said: "We can speculate that the volume reductions observed in pregnancy represent a process of specialisation or further maturation of this Theory of Mind network that, in some way, serves an adaptive purpose for pending motherhood."
The study found that pregnant women were all affected in similar ways, regardless of whether they conceived naturally or underwent IVF.
And there were no changes in first-time fathers` grey matter in the study when their brains were monitored before and after their partners` pregnancy.
The research team also found no major changes in white matter in the brain.
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