Distraction may be the problem rather than memory
Expectant mums need to stop blaming their bump for memory lapses, say experts who want to dispel the "baby brain" myth.
Neither pregnancy nor motherhood addle a woman's brain, say the researchers based on their study of 1,241 women both before and after having babies.
The Australian researchers say we have been misled by a fallacy.
Any absentmindedness might be adaptive, shifting attention to the baby, the British Journal of Psychiatry says.
Lead researcher Professor Christensen said: "Part of the problem is that pregnancy manuals tell women they are likely to experience memory and concentration problems - so women and their partners are primed to attribute any memory lapse to the 'hard to miss' physical sign of pregnancy.
"Pregnant women may also shift their focus away from work issues to help them prepare for the birth of their new baby, while new mothers selectively attend to their baby."
But she said this shift should not be labelled a "cognitive deficit".
Her team from The Australian National University followed up the large group of women at four-year intervals using memory tests.
During the course of the study more than half of the women fell pregnant, but this did not appear to have any impact on memory.
The test scores remained unchanged before and after pregnancy and did not differ greatly between the group of women who became mums and the group of those who did not.
Professor Christensen and her team said: "Not so long ago, pregnancy was 'confinement' and motherhood meant the end of career aspirations.
"Our results challenge the view that mothers are anything other than the intellectual peers of their contemporaries.
"Women and their partners need to be less automatic in their willingness to attribute common memory lapses to a growing or new baby.
"And obstetricians, family doctors and midwives may need to use the findings from this study to promote the fact that 'placenta brain' is not inevitable."
Cathy Warwick of the Royal College of Midwives said: "It is about time that some research lays to rest this notion of pregnant women and the 'baby brain' myth.
"The physical and emotional stresses on a woman's body from pregnancy can make women feel more tired than usual.
"As we all know tiredness - for men as well as women - can make us lose concentration and cause us to function less effectively.
"This is why midwives encourage pregnant women to take appropriate rest breaks, at home and at work. Many pregnant women will need this rest, and all of them deserve it."