Forgot to book that doctor's appointment? Left your daughter waiting long after her dance class finished? Dressed your son in his school uniform when today was mufti day? Well, if you're pregnant, you can legitimately blame these lapses in memory on 'baby brain', because according to a new study, that pregnancy-induced mental fog is a real phenomenon.

The ABC reports, researchers at Deakin University undertook a meta-analysis of 20 studies involving more than 1,200 women.

The research, published in the latest Medical Journal of Australia, found overall cognitive functioning was poorer in pregnant women than in non-pregnant women.

"General cognitive functioning, memory, and executive functioning (attention to detail, planning and problem solving) were significantly reduced during the third trimester of pregnancy, but not during the first two trimesters," the authors wrote.

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The study found, while changes in cognitive functioning happens between the first and second trimester, they are more obvious in the third trimester.

But pregnant mothers needn't be too worried; 'baby brain' lapses are likely to be minor, such as forgetting to book appointments, rather than impaired performance at work, or an inability to navigate complex tasks.

"It will be more a feeling they have that they're not quite as on the ball as they normally are," senior author Associate Professor Linda Byrne said.

Scientists say they're unsure why memory and some brain functions are impacted by pregnancy, and it's not yet known whether brain function returns to normal levels after birth.

Perhaps one day we will be able to blame that forgotten appointment on 'mum brain' long after we've given birth.