As many as four out of every five pregnant women say that they suffer from “pregnancy brain”—deficits in memory and cognitive ability that arise during pregnancy, making women more forgetful and slow-witted. Yet studies on the phenomenon have generally not supported these claims: although some have found evidence of problems on certain types of tasks, others, including a recent paper published by researchers in Utah, have found no signs of cognitive problems at all. Some experts believe that pregnancy brain and its postnatal cousin, “baby brain,” could largely be a product of confirmation bias: pregnant women and new moms expect to experience brain fog and therefore believe they are actually affected. Others argue that the mental symptoms might simply be too difficult to confirm in a laboratory setting.

In the most recent study, researchers at Brigham Young University gave cognitive and neuropsychological tests to 21 women in their third trimester of pregnancy and then tested them again six months after they gave birth. They administered the same tests at similar intervals to 21 women who had never been pregnant. They found no differences between the groups no matter when they were tested, including before and after giving birth. These findings mesh with those from a 2003 study, which found that pregnant women did not score differently from nonpregnant women on tests of verbal memory, divided attention and focused attention.

“There is variety in the results, but overall most studies suggest there are few to no memory impairments associated with pregnancy,” says Michael Larson, a psychologist at Brigham Young and a co-author of the recent paper. He thinks the reason the myth persists may be that women selectively look for evidence that supports the cultural expectation. For example, when a pregnant woman loses her car keys, she might blame pregnancy brain—without recalling the times she lost her car keys before she was pregnant.

Joanna Workman, a psychologist at the University at Albany, agrees that confirmation bias may play a role, but she says there is another possibility, too. In a 2011 study, a team at the University of British Columbia found that although pregnant women did not display any problems on cognitive tests given in a lab, they were less likely than nonpregnant women to remember to call the lab when asked and to return a questionnaire on time. “It's possible that lab-based measures do not reveal differences, because labs are typically quiet environments with minimal distractions, in contrast with everyday life,” she says.