"The general cognitive functioning, memory and executive functioning performance of pregnant women is significantly lower than in non-pregnant women, both overall and particularly during the third trimester," they concluded.
"The differences primarily develop during the first trimester and are consistent with recent findings of long-term reductions in brain grey matter volume during pregnancy."
The study published in the Medical Journal of Australia found memory performance declined during the early stages of pregnancy, but the decline either slowed or stopped from mid-pregnancy.
The magnitude of changes in overall cognition and memory during the third trimester is not only statistically but also clinically significant, the authors said.
The study authors say more research is needed to determine the impact of 'baby brain' on the quality of life and everyday functioning of pregnant women.
"These findings need to be interpreted with caution, particularly as the declines were statistically significant, but performance remained within the normal ranges of general cognitive functioning and memory," Associate Professor Linda Byrne said.
Dr Melissa Hayden said the small reductions in performance across their pregnancy will be noticeable to the women themselves and perhaps those close to them.
She says they mainly manifest as minor memory lapses like forgetting, or failing, to book medical appointments, but that more significant consequences such as reduced job performance or impaired ability to navigate complex tasks are less likely.