Barbara Sahakian, professor of clinical neuropsychology at Cambridge, said: "it's hard to quantify the scale of the phenomenon but it's definitely catching on."

Dr Elisabeth Martens, expert in neuropsychology at Tilburg University and lead author, said that the findings have implications for clinical practice and research.

Writing in the Journal of Clinical and Experimental Neuropsychology, Professor Larson said that the expectation of baby brain may be so strong that a pregnant woman is extra-alert to memory lapses.