Scans of 25 first-time mums showed these structural brain changes lasted for at least two years after giving birth.
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.
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.
Surface maps of GM volume changes between the Pre and Post session
Nature Neuroscience - Pregnancy leads to long-lasting changes in human brain structure
SOURCES- BBC News, Nature Neuroscience