Updated project metadata. Inherent hemispheric asymmetry is significant for cognition, language and other functions. An understanding of normal brain and asymmetry development in the early period will further the knowledge of how different hemispheres prioritize specific functions, which is still unknown. We analysed the developmental changes in and asymmetry of the proteome in the bilateral frontal lobes of three foetal specimens in the late first trimester of pregnancy (9, 11, 13 gestational weeks). We found that during this period, the difference in expression between gestational weeks increased, and the difference in asymmetric expression decreased. The patterns of protein expression changes in the bilateral frontal lobes were different. Our results show that brain asymmetry can be observed in the early stage. Researchers can use these findings to further investigate the mechanisms of brain asymmetry.