Researchers from the Australian National University examined cognitive function in 1,241 women aged 20-24. Four areas of function were assessed over eight years: cognitive speed, working memory, and immediate and delayed recall.
The researchers found no link between pregnancy and memory loss.
However, women in the later stages of pregnancy did experience a small but significant decline in cognitive speed, which reversed after childbirth.
The researchers said their findings suggested that neither pregnancy nor motherhood have a detrimental effect on a woman's cognitive capacity, a finding that directly contradicts the results of previous studies.
What lies behind the myth?
The majority of pregnancy guidebooks, the media and websites often promote the concept of a 'baby brain', suggesting that pregnancy causes women to become forgetful.
'Part of the problem is that pregnancy manuals tell women they are likely to experience memory loss,' said the lead researcher Professor Helen Christensen.
She added: '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 baby. However, this shift is adaptive and cannot be labelled a cognitive deficit.'
In addition, previous studies have shown that pregnant women perform less well than non-pregnant women on cognitive function tests, the researchers said.
However, this is the first study of its kind to recruit women prior to pregnancy, which was described by Professor Christensen as 'critical'.
'This is the first time that pre-pregnancy cognitive scores were available. We could thus see if pregnancy or motherhood produced any greater change in cognition compared with controls.'
She added: 'Our study used a representative population sample too, rather than a convenience sample.'
What advice should women be given now?
The researchers have called on GPs, obstetricians and midwives to use their findings to dispel the myth that memory loss is an inevitable consequence of motherhood.
Perceptions of cognitive impairment may reflect emotional or other, unknown factors, they said.
'Women and their partners need to be encouraged to be less automatic in their willingness to attribute common memory lapses to the salient casual factor of a growing or new baby,' they concluded.
Cathy Warwick, general secretary of the Royal College of Midwives, welcomed the findings and said that it was 'about time' the baby brain myth was challenged.
'This is useful research that I have no doubt will be interesting to women and health professionals,' she said.
'The physical and emotional stresses on a woman's body from pregnancy can naturally make women feel more tired than usual.
'As we all know tiredness - for men as well as women - can naturally make us lose concentration and cause us to function less effectively.'
She encouraged pregnant women to be aware of 'over-doing it' and to take appropriate rest breaks to avoid tiredness and poor concentration.
'Many women will need this rest, and all of them deserve it,' she said.