A new study has verified something women have suspected for a long time - that pregnancy can really mess with a young mum's cognitive function.

According to a new meta-analysis conducted by researchers at Deakin University, forgetfulness and other cognitive deficiencies during pregnancy are actually measurable - but the impact is likely only noticed by the pregnant woman herself and those closest to her.

Also sometimes called "mumnesia", baby brain often manifests as a sort-of escalation of everyday absentmindedness - more often forgetting where you may have put something, or losing track of what you're doing mid-task.

Women have also reported losing track of conversations, needing to rely more heavily on note-taking to stay organised, difficulties with reading comprehension, and even going so far as to having to delay returning to work.

But attempts to study the phenomenon have yielded inconsistent results, the researchers said, with some studies finding the changes minor. Even a 2007 meta-analysis, now over 10 years old, found that the effect is inconsistent across different areas of memory.

The new meta-analysis of 20 studies updates the body of research regarding the phenomenon by including studies conducted more recently than 2007, as well as examining the cognitive effects on areas other than memory.

The studies analysed included a total of 1,230 women - 709 were pregnant, and the remaining 521 were non-pregnant control groups. They all also included at least one standard objective measure of cognitive function.

Overall, the researchers found, cognitive function was definitely poorer in pregnant women compared to women who weren't pregnant.

"General cognitive functioning, memory, and executive functioning were significantly reduced during the third trimester of pregnancy (compared with control women), but not during the first two trimesters," the authors wrote in their paper.

"Longitudinal studies found declines between the first and second trimesters in general cognitive functioning and memory, but not between the second and third trimesters."

Overall, declines in cognitive function develop during the first trimester, and are consistent with recent research that found a physical reduction in grey matter during pregnancy.

"It looks like the reason pregnant women have grey matter reduction is because they're probably recruiting those areas to more important areas associated with the business of child rearing - so things like bonding, and social cognition," researcher Linda Byrne told the ABC.

Further research will be needed to find out the impact of this on pregnant womens' quality of life, but the researchers noted that, even with a measurable decline, the women's cognitive and memory function still remained within a very normal and functional range.

However, establishing some of the more serious effects could help develop methods for helping the small minority of women affected by them.

"These small reductions in performance across their pregnancy will be noticeable to the pregnant women themselves and perhaps by those close to them, manifesting mainly as minor memory lapses (e.g., forgetting or failing to book medical appointments)," says senior researcher Melissa Hayden.

"But more significant consequences (e.g., reduced job performance or impaired ability to navigate complex tasks) are less likely."

The research has been published in the Medical Journal of Australia.