Background: Prenatal physical activity is associated with numerous maternal and child health outcomes. Unfortunately more than half of US women fail to engage in regular exercise during pregnancy. Thus there is a clear need for effective intervention strategies aimed at increasing physical activity among pregnant women. In order to meet this need, the present study provides a systematic review of prospective and intervention studies to identify potential mediators of physical activity during pregnancy. Methods: A systematic review was conducted searching PubMed, MEDLINE, CINAHL, and PsychINFO for prospective studies published from 1995 through 2012 that examined potential mediators to physical activity as well as any intervention studies that sought to increase physical activity levels during pregnancy. Eleven prospective studies that examined potential mediators to physical activity and 3 intervention studies that aimed to increase physical activity during pregnancy were selected that met the inclusion criteria of this review. Articles were reviewed and discussed according to the population, study design, mediators examined (e.g. beliefs pertaining to perceived benefits, barriers, severity and self-efficacy as well as social support, behavioral skills, subjective norms, and behavioral intention), measured outcomes, results, and conclusions of the studies. Results: The review identified several significant mediators of perinatal physical activity such as beliefs (e.g. perceived benefits, barriers, severity and self-efficacy toward physical activity), attitudes, social support, and perceived stress, among others. Conclusions: This review highlights psychosocial mediators that show promise in influencing physical activity behavior. Given that there are very few intervention studies that target these psychosocial mediators more research is warranted in this area.