Up to 81 per cent of women who have been pregnant report "baby brain", or subjective symptoms including reading difficulties, absent-mindedness and forgetfulness.

PhD candidate Sasha Davies and colleagues at Deakin University undertook a meta-analysis of 20 studies that included 709 pregnant women and 521 women who were not pregnant.

The research, published in the latest Medical Journal of Australia, found the evidence from the studies indicated that "overall cognitive functioning was poorer in pregnant women than in non-pregnant women".

"General cognitive functioning, memory and executive functioning were significantly reduced during the third trimester of pregnancy (compared with control women), but not during the first two trimesters," the authors wrote.

Four longitudinal studies found declines between the first and second trimesters in general cognitive functioning and memory, but not between the second and third trimesters.

The authors said the differences developed primarily during the first trimester and were consistent with recent findings of long-term reductions in brain grey matter volume during pregnancy.

"The impact of these effects on the quality of life and everyday functioning of pregnant women requires further investigation," they wrote.

But Associate Professor Linda Byrne said the findings needed to be interpreted with caution, particularly since while the declines were statistically significant, performance remained within the normal ranges of general cognitive functioning and memory.

Co-author Melissa Hayden said the small reductions in performance across pregnancy would be noticeable to the pregnant women themselves and maybe people close to them.

These could manifest as mainly minor memory lapses, such as forgetting or failing to book medical appointments.

"But more significant consequences, for example reduced job performance or impaired ability to navigate complex tasks, are less likely," Dr Hayden said.