A study by the Australian National University's centre for mental health research found that there is no evidence to suggest that impending motherhood affects a woman's cognitive ability.
The research is based on analysis of interviews with 2,500 women aged between 20 and 24 first undertaken in 1999 and again later in 2003 and 2007.
It found that the 76 women who were pregnant during the second or third interviews scored no differently on logic and memory tests than previously.
"And there were no differences between them and control women," Professor Helen Christensen, who led the research, told AFP.
"It really leaves the question open as to why (pregnant) women think they have poor memories when the best evidence we have is that they don't."
Christensen said while it was possible the tests were unable to pick up subtle changes in the brain, it seemed more likely that women blamed pregnancy for minor lapses because it was foremost in their mind at the time.
"It makes it very easy to attribute what might be just normal lapses in memory to pregnancy," she said.
The professor said research on rodents had found that mother rats had an improved capacity to do more than one task, navigated mazes more efficiently and suffered less anxiety and fear.
"There's enormous changes in the rat brain during pregnancy so you might actually expect that women perform better during pregnancy than when they're not pregnant," Christensen said.