And according to a researcher, that's a good thing, because it means your brain cells are changing to focus more on your unborn child and to prepare you for motherhood.
Laura Glynn, a psychologist at Chapman University, California, claims mothers-to-be suffer short-term memory loss as a result of massive fluctuations in their hormones as well as tiny movements by the foetus.
She suggested women's brains change during pregnancy so that they will be better able to concentrate on their newborn's needs after the birth.
Dr Glynn said that there "may be some cost" of these changes – such as absent-mindedness – "but the benefit is a more sensitive, effective mother".
She also said that just the slightest movement of the foetus in the womb can affect a woman's brain and make her become more sensitive.
She said that even though the woman may not be aware of these movements they will raise her heart rate.
"Pregnancy is a critical period for central nervous system development in mothers, yet we know virtually nothing about it," said Dr Glynn, whose research is published in the journal Current Directions In Psychological Science.
She also said that cells from the foetus will pass into the mother's bloodstream which will also affect the way her brain works.
Did you have 'baby brain' or do you think it's a myth and you're just bone tired from pregnancy?