After forgetting to thank her fiancé in her Oscar speech, Rachel Weisz, the actress, blamed it on her pregnancy, saying her brain was "like porridge".

But "cottonwool brain" among expecting mothers may simply be a social myth, research shows. Psychologists did find pregnant women were slightly slower at some tasks, but the differences were so mild they put them down to slight hormonal changes or even the subconscious assumption by the women that their abilities had been impaired.

Dr Ros Crawley, from the University of Sunderland, tested 25 women in the third trimester of pregnancy, 24 women in the second and 25 who were not pregnant on 14 tasks, including remembering information, hunting a phone number and following progress of a lift between floors.

She reported in the journal Psychology and Psychotherapy that there was no difference in 11 tasks, but in two - speed of comprehension and following the lift - the pregnant groups were slower. But when pregnant women, their partners and other men and women were asked about expecting mothers, all believed pregnancy did affect memory, attention and the ability to perform actions such as driving a car.