Introduction: acquisitions and changes in the motor and cognitive development of boys and girls are related not only to existing biological differences between both sexes, but also to socio-economic, cultural and family factors. Objective: to investigate the differences between sexes in the acquisition of anti-gravitational postures. Methods: the participants in this study were 638 children born at term (324 males and 314 females), from 0 to 18 months, coming from Infant Education Schools in the south of Brazil. The Alberta Infant Motor Scale (AIMS) was used to evaluate motor performance. Results: most of the evaluated children showed normal motor performance for their age (69.7%), with nonlinear development and plateaus in postural acquisition from 15 months. There were not significant differences (p>0.05) in motor performance between boys and girls from 0 to 18 months. Conclusion: motor development was similar between the sexes in the first months of life. However, throughout childhood, sociocultural differences and parents’ practices seem to influence differently the process of motor acquisition and development of skills, since children are exposed to experiences in conformity with sex expectations
