Research examining the motivational determinants of exercising during pregnancy is mostly atheoretical, despite the need for theory-based designs. The study's main objective was to prospectively examine women's exercise intention and behavior from their second to third pregnancy trimester using the theory of planned behavior (TPB). Participants were 89 pregnant women who completed self-reported measures of their exercise attitude, subjective norm, perceived behavioral control, intention, and behavior. Hierarchical regression analyses indicated: 1) intention and not perceived behavioral control significantly predicted exercise behavior; and 2) attitude was the strongest determinant of exercise intention, followed by perceived behavioral control, and subjective norm. The study findings provide preliminary support for the TPB as an effective framework for examining exercising during pregnancy. Understanding women's thoughts, feelings, and beliefs about exercise can assist intervention specialists with developing and implementing effective programs promoting exercise during pregnancy.