Psychological factors in pregnancy and mixed-handedness in the offspring.

Animal studies suggest that psychological factors may interfere with the development of brain asymmetry during gestation. We evaluated whether psychological exposure in pregnancy was associated with mixed-handedness in the offspring. In a follow-up design study, 824 Danish-speaking women with singleton pregnancies provided information on psychological distress and the occurrence of life events in the early second and third trimesters of pregnancy. Handedness of the children was based on maternal reports when the children were 3 years of age. Among the 419 males and 405 females, 7% and 5% respectively were mixed-handed whereas mixed-handedness was found in 3% of the parents. Psychological distress in the third trimester as well as higher levels of stressful life events were related to a higher prevalence of mixed-handedness in the offspring. About 16% of the women reported more than one life event in the third trimester of pregnancy and among the offspring of these women 11% were mixed-handed (odds ratio = 2.3; 95% confidence interval 1.2 to 4.4). Women who at the same time reported a high level of distress and stressful life events, had a three- to four-fold higher prevalence of mixed-handedness in their offspring.