The purpose of this study was to investigate the association between infant birth weight and standing at work during pregnancy. A total of 1222 pregnant women employed in a large petrochemical corporation in Beijing, China, were enrolled in the study, after receiving permission from the government to have a child. The subjects were followed up from that time through their entire pregnancy, for a total of up to 12 months. All subjects delivered at the company staff hospital between 1996 and 1998. Various work-related physical activities during pregnancy were assessed using a structured questionnaire, and generalized additive models (GAMs) were performed to examine their association with birth weight. Of the assessed activities, only standing was significantly associated with birth weight. After adjusting for potential confounders, maternal standing hours per day at work was found to be significantly associated with reduced birth weight (-17.7 g, P = 0.03).